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Credit…Tamir Kalifa for The New York Times

The U.S. surpasses 50,000 new cases in a single day for the first time.

More than 53,000 new coronavirus infections were reported across the United States on Thursday, according to a New York Times database, as the country set a new daily case record for the sixth time in nine days. The alarming new milestone came as some of the country’s most populous states reported major surges, and as public health officials scrambled to limit the damage.

At least eight states reported single-day case records on Thursday: Alaska, Arkansas, California, Florida, Georgia, Montana, South Carolina and Tennessee.

Thursday’s reported infection total represented an increase of more than 85 percent in daily cases since two weeks ago, when states were reopening after extensive lockdowns eased the outbreak, particularly in the hard-hit Northeast. Until last week, the country had not surpassed a record daily total for two months, since 36,738 new cases were reported on April 24.

Now the surge in cases has shifted to the South and the West, as the nation staggers toward a holiday weekend burdened by a pandemic that threatens to keep growing worse.

On Tuesday, as the outbreak expanded, Dr. Anthony S. Fauci, the country’s leading expert on infectious diseases, warned that new infections could rise to 100,000 a day as the outbreak resumed its march.

On June 1, Florida officials announced 667 new cases. On July 1, they added more than 6,500. On Thursday, they reported over 10,100, a record.

In California, where some newly reopened businesses were again being shut down, new case reports went from about 2,500 to more than 8,200 on Thursday, for the state’s highest single-day total.

If June was the month when the pandemic spiraled from America’s grasp, July seems destined to be month when the country will learn just how bad it will get.

Although Trump administration officials have argued in recent weeks that the rise in cases can be attributed to increased testing, Adm. Brett P. Giroir, the assistant secretary of health who is overseeing the government’s testing response, told Congress on Thursday that the higher numbers in fact reflect an increase in cases.

“There is no question that the more testing you get the more you will uncover, but we do believe this is a real increase in cases because the percent positivities are going up,” Admiral Giroir said. “So this is real increases in cases.”

Ohio, Kansas and Louisiana, all of which looked stable not long ago, posted some of their highest single-day totals in weeks.

Though single-day snapshots are an imperfect measure of the pandemic, the broader picture is also exceedingly bleak. Case numbers were trending upward in 38 states as of Wednesday. The problem spots in the country’s South and West were spreading north and east, and hospitalizations surged in some states.

Pence changes his travel plans after Secret Service agents tested positive for the virus.

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Vice President Mike Pence changed his travel plans in Arizona after Secret Service agents set to accompany with him tested positive for the coronavirus or showed symptoms, two administration officials said on Thursday.

Mr. Pence had been scheduled to visit Arizona on Tuesday, but multiple factors related to the spread of the virus foiled those plans, according to a person familiar with Mr. Pence’s travel.

A swift rise in new cases in the state has overwhelmed testing centers in recent days, and Gov. Doug Ducey, a Republican, ordered bars, gyms and movie theaters closed this week. In an apparent acknowledgment of outbreaks erupting across the South and the West, the vice president canceled his plan to headline a “Faith in America” campaign rally in Tucson on Tuesday and then tour Yuma with Mr. Ducey.

Instead, Mr. Pence opted for a shorter visit to Phoenix on Wednesday, where he participated in a public health briefing at Sky Harbor International Airport.

“Help is on the way,” Mr. Pence said at a news conference with Mr. Ducey at the airport, after descending the steps of Air Force Two wearing a mask, the latest sign of the administration’s evolving stance on face coverings.

But the positive tests and symptoms of Secret Service agents expected to be in proximity to the man who is second in line for the presidency were some of the factors that prompted his change of schedule, the officials said. The news of the agents who showed symptoms of Covid-19, or tested positive, was first reported by The Washington Post.

A spokeswoman for Mr. Pence did not respond to a request for comment.

The latest illnesses among the small circle of individuals who interact directly with the vice president were a reminder of the dangers of carrying on with campaign and official government travel as the pandemic rages on.

The governor of Texas orders most residents to wear masks in public.

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Governor Orders Texans to Wear Masks As Virus Cases Surge

Gov. Greg Abbott ordered residents to wear face coverings in public as the state confronts mounting coronavirus infections and pressure from local officials.

In the past few weeks, there has been a swift and a substantial spike in Covid-19 cases. So we need to refocus on slowing the spread. But this time we want to do it without closing down Texas again. One of the best ways to keep businesses open while also slowing the spread is for everyone to wear a face covering like this when they go out. Medical studies have shown that wearing a face covering slows the spread of Covid-19 and it protects you and your family. That is why today, I am issuing a face-covering requirement for all counties with more than 20 Covid cases. This safe standard requires Texans to wear masks in public spaces with certain common sense exceptions. Local law enforcement has the authority to enforce this safety standard just like they do when enforcing seat belt standards. Now I know that wearing a face covering is not the convenient thing to do. But I also know that wearing a face covering will help us to keep Texas open for business. And it will help Texans earn the paycheck they need.

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Gov. Greg Abbott ordered residents to wear face coverings in public as the state confronts mounting coronavirus infections and pressure from local officials.CreditCredit…Tamir Kalifa for The New York Times

Gov. Greg Abbott of Texas issued a sweeping order on Thursday requiring most Texans to wear masks in public, in an abrupt reversal as cases soar in the nation’s second-largest state. The order applies to Texas counties with 20 or more coronavirus cases.

The order represents a stark change for Mr. Abbott, a Republican, who earlier opposed attempts by Democratic mayors and other local officials to require everyone in their cities to wear masks in public. But as cases surged in recent weeks, he cleared the way for local authorities to require masks in businesses, before imposing the more aggressive statewide requirement on Thursday.

“We have the ability to keep businesses open and move our economy forward so that Texans can continue to earn a paycheck,” Mr. Abbott said in a statement. “But it requires each of us to do our part to protect one another — and that means wearing a face covering in public spaces.​”

On Wednesday, Texas reported a record of more ​​​​than 8,000 new cases, then about 7,300 on Thursday. The outbreak there, and in other states across the South and West, has fueled record numbers of new cases in the United States. The nation reported nearly 50,000 new cases on Wednesday, a record. And at least six states reported single-day case records on Thursday: Arkansas, Florida, Georgia, Montana, South Carolina and Tennessee.

Mr. Abbott’s order has limited exceptions for young children, people with medical conditions and people eating in restaurants or exercising outdoors.

Mr. Abbott had previously cast his opposition to mask mandates as a matter of principle. Last month, in an interview with KWTX-TV, Mr. Abbott said that while businesses could be ordered to require masks, “we want to make sure individual liberty is not infringed upon by government, and hence government cannot require individuals to wear masks.”

The governor also said that with limited exceptions Texans cannot gather in groups larger than 10 people and must maintain six feet of social distance from one another.

Even the aging rapper Vanilla Ice has yielded to pleas from local health officials, backing off a planned July 4 concert at Lake Travis in Texas. He said on Thursday that he “didn’t know the numbers were so crazy.”

Gov. Abbott’s decision was the latest sign that Republicans are embracing masks, as the virus spreads in a number of Republican-led states in the South and the West. Vice President Mike Pence pointedly wore a mask at a church visit in Dallas on Sunday. Even President Trump, who has resisted wearing a mask in public, said this week, “I’m all for masks,” while still declining to wear them.

The Supreme Court grants Alabama’s request to restore voting restrictions during the pandemic.

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By a 5-to-4 vote, the Supreme Court on Thursday blocked a trial judge’s order that would have made it easier for voters in three Alabama counties to use absentee ballots in this month’s primary runoff election.

In March, Gov. Kay Ivey, a Republican, postponed the election because of the pandemic. At the same time, the Republican official who oversees the state’s elections, John H. Merrill, Alabama’s secretary of state, expanded the availability of absentee ballots to all voters who concluded that it was “impossible or unreasonable to vote at their voting place.”

But Mr. Merrill did not relax two of the usual requirements for absentee voting: submission of a copy of a photo ID with a voter’s application for a ballot and submission of an affidavit signed by a notary public, or two adult witnesses, with the ballot itself.

Four voters and several groups sued to challenge those restrictions, saying that, in light of the health crisis, they placed an unlawful burden on the right to vote. Making a copy of a piece of identification, for instance, may be difficult and dangerous during the pandemic, they said.

Officials in Alabama, which has more than 40,000 confirmed coronavirus cases and nearly 1,000 deaths, have dismissed that concern.

In asking the Supreme Court to intervene, state officials said an order by an Alabama judge — which, among other things, blocked election officials in two counties from enforcing the ID requirement for voters who are disabled or 65 or older — had come too close to the election and threatened its integrity.

In their brief, the state officials discussed ways in which voters could safely comply with the witness requirement.

In response, lawyers for the voters said the state had offered no good reasons “to justify the application of the witness or photo ID requirements to high-risk voters in the middle of a pandemic.”

Trump’s problems include a disconnect between a case surge and his dismissive stance.

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As President Trump heads to Mount Rushmore on Friday to spend the Independence Day holiday in the carved presence of presidential greatness, he is suffering through the most trying stretch of his administration thanks in large part to his self-inflicted wounds.

Last month’s convergence of crises, and the president’s missteps in responding to them, have been well-chronicled: his inflammatory response to racial justice protesters and his ill-considered rally in Tulsa, his refusal to acknowledge the resurgent virus or seriously address detailed reports about Russian operatives’ putting a cash bounty on American soldiers. It’s this kind of behavior, polls indicate, that has alienated swaths of swing voters.

The disconnect between the surge in coronavirus cases and Mr. Trump’s dismissive stance toward the pandemic has been particularly pronounced, mystifying Democrats and Republicans alike; this week, as some states halted their reopening because of a record-setting number of new cases, the president predicted the virus would “just disappear.”

What mystifies many Republicans about Mr. Trump is why he is so unwilling to take easy steps that could help remedy his political difficulties. The most visible example is Mr. Trump’s refusal to promote mask-wearing to fight the virus, which poses perhaps the most dire threat to his re-election.

Mr. Trump continues to hope for an economic recovery he can run on in the final four months of the campaign, and on Thursday he trumpeted as a sign of progress the employment report showing 4.8 million jobs gained in June. But it is not clear that Mr. Trump will get much credit for a partial — and possibly fleeting — rebound when coronavirus cases are soaring.

Global roundup

The pandemic crushed Venice’s tourism industry. Not everyone minds.

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Credit…Andrea Merola/EPA, via Shutterstock

Long before Venice became a hot spot for international travelers, locals would practice the tradition of flânerie, an aimless stroll through the city’s calli, or walkways.

Now flânerie is back: The pandemic has crushed the tourism industry, and many residents have more time and space to enjoy the city’s slow pace and faded beauty, even if money is tight.

Venice is one of several European cities that rely heavily on tourism and was particularly vulnerable to pandemic’s economic side effects. Some economists have even described it as a “tourism monoculture,” borrowing the term from the risky agricultural practice of growing a single crop.

Some residents and officials now hope that city’s economy can begin to move away from that monoculture — for example, by drawing in international investors and expanding the footprint of its two universities.

“We have to act now, before mass tourism will be back at full capacity, because we won’t get a second chance,” said Paolo Costa, an economics professor and former mayor of Venice.

  • Officials in Tokyo, Japan, said on Friday that the number of new cases had topped 100 for a second straight day, according to the national broadcaster NHK. The spike came nearly six weeks after the Japanese capital lifted a state of emergency and declared the virus contained.

  • In recent weeks, Brazil has emerged as one of the world’s most severe hot spots, second only to the United States. With Brazil’s caseload ballooning, Ernesto Londoño, the Brazil bureau chief for The New York Times, explains what went wrong.

Listen to ‘The Daily’: What Went Wrong in Brazil

Despite a history of success in public health crises, the country has one of the world’s worst coronavirus outbreaks.

  • The European Centers for Disease Control and Prevention urged European Union countries to step up testing and contact tracing, and asked governments to communicate to their citizens that “the pandemic is not over.” The warning comes as Europe’s reopening has brought a resurgence of cases in some pockets.

  • Officials in Britain failed to heed early warnings that domestic abuse would soar during the lockdown. For some victims trapped with their abusers, the consequences have been catastrophic.

  • As the number of coronavirus infections soars in some countries, the mortality rate has not kept pace, and some believe the growing number of young people on the case list may account for that disparity. But the chief scientist of the World Health Organization noted Thursday that young people are vulnerable to severe complications from the virus.

  • Days after a wedding in India, the groom died, and at least 100 guests tested positive for the virus. Indian officials have opened an investigation into the mid-June gathering, which some experts are calling a superspreader event, attended by more than 300 guests in the city of Patna, in the northeastern state of Bihar.

Congress eyes more spending as the virus surges and the economy falters.

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There is a growing recognition across party lines that Congress will need to spend more money, soon, to continue to prop up the American economy.

But there is little consensus on what the next aid package should look like and how quickly it will arrive.

Some economists say lawmakers are risking further damage to an already fragile recovery by not moving more quickly. Real-time indicators of shopping patterns and business openings suggest that a once-brisk economic rebound stalled in June as the virus began spreading more rapidly in Texas, Florida and other states. And even the most encouraging signs of recovery — such as the jobs report released today, which said U.S. payrolls grew by 4.8 million in June — underscore how far the recovery still has to get back to what was normal before the virus: Nearly 18 million Americans remain unemployed.

Lobbyists and lawmakers say the Trump administration, which has lost several economic advisers in recent weeks, is not deeply engaged in devising another rescue package. Officials have hinted for weeks that they would formally propose tax cuts, infrastructure spending and other initiatives, but they have not followed through. Mr. Trump has expressed support for additional tax cuts and government spending.

Senators are expected to leave Washington on Thursday after making only incremental progress toward an agreement to extend further relief to businesses and laid-off workers who are about to lose or have already exhausted federal assistance.

“The stimulus was very short-lived,” said Aneta Markowska, chief economist at the investment bank Jefferies. “This problem is going to persist long beyond July.”

Only a dozen states have managed to keep new case levels flat or declining.

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Credit…Charles Krupa/Associated Press

As the virus has exploded in large swaths of the nation over the past two weeks, only a dozen states, mostly in the Northeast, have managed to keep new case levels flat or declining.

New York, which averaged 9,877 cases per day when it reached the height of its outbreak in April, has been staying mostly flat at an average of 620 cases per day. Other states in the Northeast that were initially hit hard, including New Jersey, Connecticut, Massachusetts and Rhode Island, are all plateauing at far lower levels than earlier in the pandemic.

Massachusetts announced Thursday that it would move forward with the next phase of its reopening on Monday for all areas of the state except for Boston.

Movie theaters, outdoor performance venues, fitness centers, museums and other businesses will be allowed to resume operations. Up to 25 people will be allowed to gather in indoor spaces, provided that they are adequately spaced. Boston is scheduled to ease its restrictions in those areas on July 13.

“Today the public health data makes clear that Massachusetts is effectively bringing the fight to the virus as we’ve reopened,” Gov. Charlie Baker said at a news conference Thursday.

Other states where new cases have stayed largely the same over the past two weeks, according to a New York Times database, include Vermont, Maryland, Virginia, Indiana, Nebraska and South Dakota.

The state showing the biggest decline in new cases over the past two weeks, according to the database, was New Hampshire. Its decline has been modest: It is now averaging 29 new cases a day, down from over 34 cases a day two weeks ago.

But its governor, Christopher T. Sununu, said that the state should not expect its declines to last.

“I actually think a spike in Covid cases will happen in New Hampshire, and that is the real tough situation that we have,” he said Wednesday on WMUR. “It would be very ignorant and arrogant of us to think that we are going to be completely immune to what’s happening in other parts of the country.”

“The reason the spikes are happening in other parts of the country aren’t just because they’re reopening,” he said, adding that many people in other areas, and particularly young people, were not wearing masks or practicing social distancing.

“Unfortunately I think we are going to see some outbreaks here,” he said. “The key is here we can manage it this time: We have P.P.E., we have testing capabilities, we have more financial resources.”

The mayor and the governor sow confusion about reopening New York City’s schools.

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Credit…Juan Arredondo for The New York Times

Mayor Bill de Blasio said Thursday that New York City schools would open in some form come September — only for Gov. Andrew M. Cuomo’s office to describe the mayor’s announcement as “premature.”

“We value the opinion of local politicians and the state’s 700 local school districts as to what should be done, but the public should not be confused on this important decision that has practical consequences for many,” Dani Lever, Mr. Cuomo’s spokeswoman, in a statement.

The lack of coherent communication from the state and the city about the nation’s largest school system sowed further confusion among public school parents eager for clarity about the coming school year. Some private schools are also are likely to follow the public school path.

It is not the first time the two officials have differed on schools. In April, when Mr. de Blasio announced that they would remain closed for the rest of the school year, Mr. Cuomo discounted it as merely the mayor’s “opinion.”

For months, Mr. de Blasio has suggested that schools would reopen in some form in September, On Thursday, he offered slightly more clarity.

The mayor said that students and teachers would be required to wear masks, that schools would get daily deep cleanings, and that each school would welcome the maximum daily number of students possible, given social distancing and other health requirements.

He cited a recent survey of public school parents finding that 75 percent of them want their children to return to class this year. The survey got 400,000 responses.

On Thursday, the schools chancellor, Richard A. Carranza, met with principals to discuss plans for reopening. The schools will be able to reopen only with buy-in from unionized teachers and staff members.

The teachers union president, Michael Mulgrew, said he anticipated the city would release its models for “blended” virtual and in-person learning next week.

“Our final decision about moving ahead in September depends upon medical and scientific assessments concerning the virus and its containment,” Mr. Mulgrew wrote in a Thursday email to members. “The changing nature of these assessments means we must wait to make that decision closer to the first day of school even though we know that delay creates difficulties for parents and educators alike.”

U.s. roundup

Florida reports over 10,100 new cases. Contact tracers are feeling overwhelmed.

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Credit…Eve Edelheit for The New York Times

As Florida reported on Thursday more than 10,100 new cases, breaking another single-day record, Miami-Dade Mayor Carlos A. Gimenez said that he was imposing a countywide curfew from 10 p.m. to 6 a.m., starting Friday and until further notice.

“This is one of various actions I’m pursuing to tamp down this spike of Covid-19 and protect our residents,” he said in a statement. He is also rolling back the opening of movie theaters, arcades, casinos, concert halls, bowling halls and adult entertainment venues that recently had their plans approved by the county.

“People going to restaurants will also have to keep their masks on while seated at their table — removing the mask only to eat and drink,” he said.

Miami-Dade and Broward counties had already announced they were closing beaches for the busy Fourth of July weekend.

Florida reported Thursday that an 11-year-old boy in Miami-Dade County had died, the youngest death attributed to Covid-19 in the state.

Case numbers had been lower earlier in the week, but that may have been because fewer test results were being reported each day. Nearly 69,000 results were reported on Thursday, compared with about 45,000 on Wednesday.

Vice President Mike Pence visited the state Thursday and met with Gov. Ron DeSantis to discuss the state’s rapidly worsening outbreak. The visit was quite different in tone from one in May, when the two men ate, unmasked, at a burger bar and Mr. Pence praised the state’s reopening plans, saying, “I really think Florida has set the pace.” Back then Mr. DeSantis proclaimed “we’ve succeeded,” and added, “I think that people just don’t want to recognize it because it challenges their narrative.”

On Thursday, both men wore masks, but removed them to speak at a news conference, and Mr. Pence said that the federal government would work with the state to help flatten its curve. “The economic comeback that is underway is a demonstration that we don’t have to choose between opening up America and the health of our people: We can do both,” he said. “And that’s the challenge that we face today across the Sun Belt.”

Contact tracers, unable to keep up with so many new cases, have become overwhelmed by the surge.

“We are starting to lose the ability to do contact tracing,” said Dr. J. Glenn Morris, director of the Emerging Pathogens Institute at the University of Florida.

City and county officials desperate to stop the spread have adopted local mask mandates and closed the beaches over the holiday weekend. Some communities are deploying teams of public employees to go door-to-door in the hardest hit neighborhoods, distributing masks, hand sanitizers and fliers with information on symptoms and testing.

Testing demand has remained high: In Miami-Dade County, which has averaged more than 1,400 daily cases, the biggest testing sites begin taking appointments at 9 a.m. for the next day and have filled them by 9:30 a.m., county officials said. “The requests for testing have exploded,” said Maurice Kemp, county deputy mayor.

Elsewhere in the United States:

  • When South Carolina’s coronavirus case counts started to rise sharply in June, the Citadel, one of the few state-supported military colleges in the country, rolled out an elaborate plan for the football team to work out safely over the summer. But 13 players tested positive anyway, cutting the team’s training short. They were exposed by attending an off-campus party, or by living with someone who had, according to Col. Colonel John Dorrian, the college’s vice president for communications and marketing.

  • N.F.L. team owners and the players’ union, mindful of other professional leagues’ struggles to return to play, are considering playing the 2020 regular season with few or no fans in attendance. They are also discussing how to absorb lost revenue potentially worth billions, according to three people briefed on the discussions who spoke on the condition of anonymity because they were not allowed to discuss the matter publicly.

  • Five of the nine Atlantic City casinos were reopening Thursday but with new rules: Gamblers must wear masks, and won’t be allowed to smoke, drink or eat inside.

  • In New York City, Mayor Bill de Blasio announced on Thursday that the vast majority of families have indicated that they want to send their children back to school in September, according to a survey of roughly 450,000 families. The city is still working on plans to safely maximize the number of students in schools, he said, adding that schools could shift to a staggered schedule. The governor has said logistics are up to individual districts.

  • Also in New York City, 22 streets, some already closed to car traffic, will be dedicated to outdoor dining on Friday nights and weekends. The move will be key in “some places in the city where we have extraordinary restaurants concentrated in one place,” the mayor said.

  • Officials rushing to contain a virus cluster tied to a party in a New York City suburb used an unusual legal strategy: issuing subpoenas to partygoers.

  • Herman Cain, a former Republican presidential candidate, announced Thursday that he had been hospitalized with the virus. Mr. Cain, who attended Mr. Trump’s indoor rally in Tulsa, Okla., on June 20, learned Monday that he had tested positive, was hospitalized Wednesday and on Thursday “was resting comfortably in an Atlanta-area hospital,” according to a statement posted on his Twitter account.

California’s governor again implores residents to wear masks.

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Newsom Urges Californians to Avoid Large Holiday Events

Gov. Gavin Newsom introduced a public awareness campaign about the importance of wearing masks to flatten the curve of the coronavirus, and urged residents to refrain from large gatherings this holiday weekend.

The evidence is simply overwhelming: Masks keep Californians healthy. We are now launching a major public awareness campaign — announcing it formally today, launching it formally today — that is much more comprehensive, much more dynamic, we hope much more impactful, to encourage people to wear masks and mitigate the spread of this virus. Wearing a mask is foundational, and again, it’s mandatory here in the state of California. We don’t want to see you, a bunch of strangers, without being physically distanced and without wearing face coverings over the weekend, to the extent you can. That’s why we say avoid crowds and avoid going to large parades outside of your household.

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Gov. Gavin Newsom introduced a public awareness campaign about the importance of wearing masks to flatten the curve of the coronavirus, and urged residents to refrain from large gatherings this holiday weekend.CreditCredit…Bryan Denton for The New York Times

Noting what was probably his last chance to reach residents before the July 4 holiday, Gov. Gavin Newsom of California on Thursday implored residents to refrain from gathering with people outside their household — no matter how great the temptation — and to wear masks.

“The evidence is simply overwhelming,” he said in a video news conference. “Masks keep Californians healthy.”

Mr. Newsom unveiled a new public awareness campaign about the importance of wearing masks, having already ordered people to wear them in most indoor — and some outdoor — public settings.

But amid rising case counts and hospitalizations, the governor was peppered with questions about how the state will actually enforce newly announced restrictions across counties where local leaders have shown varied willingness to comply with state-issued directives.

That particularly applied to 19 counties, home to almost three quarters of the state’s population, where the spread of the virus has been the most disturbing.

“We are working with counties in the spirit of partnership,” Mr. Newsom said. “But if we don’t see the level of local action that we are hoping for, with all 19 coming into compliance, that hand will be forced.”

On Wednesday, he announced that he was establishing teams to work with local authorities. The governor has emphasized that $2.5 billion in pandemic aid will be reserved for counties that do their share of enforcement. Nevertheless, he said, “it requires some level of personal responsibility.”

“The most important thing we’ve learned over the past couple of months is that we should focus on how, not when, to safely reopen,” Mr. Newsom said.

The state on Thursday reported thousands of new cases, he said. Over the past two weeks, he said, the rate of positive tests had increased to 6.3 percent, as hospitalizations increased by 56 percent and intensive care cases rose by 49 percent.

Researchers find new evidence that a mutation helped the virus spread, but questions linger.

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Credit…National Institute of Allergy and Infectious Diseases, via Reuters

For months, scientists have debated whether a variant of the virus that has come to predominate in much of the world did so partly because it is more transmissible than other viruses. On Thursday, a team of researchers reported new evidence suggesting that the variant did have such an advantage.

The new paper, led by Bette Korber, a theoretical biologist at Los Alamos National Laboratory, and posted by the journal Cell, presents evidence in the form of lab findings, tests of infected patients and a broad statistical analysis of the pandemic as the D614G variant took over in cities, regions and countries. “The consistency of this pattern was highly statistically significant, suggesting that the G614 variant may have a fitness advantage,” the authors concluded.

The underlying question is important for understanding the early phases of the pandemic and anticipating how it will progress in the coming months. If the genetic glitch that defines the variant imparted even a slight increase in transmissibility, it would help explain why infections exploded in some regions and not in others with similar density and other attributes.

Skeptics argue that it is far more likely that the variation spread widely by chance, multiplying outward from explosive outbreaks in Europe.

“This is an extraordinarily challenging problem,” said Dr. Marc Suchard, a biostatistician at the University of California, Los Angeles, School of Medicine. “The evolution and demography are complex. So there’s much more work to be done.”

Reporting was contributed by Rachel Abrams, Livia Albeck-Ripka, Aliza Aufrichtig, Julie Bosman, Alexander Burns, Benedict Carey, Ben Casselman, Stephen Castle, Emily Cochrane, Michael Cooper, Jill Cowan, Steven Erlanger, Richard Fausset, Luis Ferré-Sadurní, Kimiko de Freytas-Tamura, Thomas Fuller, Jenny Gross, Maggie Haberman, Jack Healy, Makiko Inoue, Annie Karni, Isabella Kwai, Ernesto Londoño, Jonathan Martin, Patricia Mazzei, Mark Mazzetti, Giulia McDonnell Nieto del Rio, Jesse McKinley, Sarah Mervosh, Anna Momigliano, Monika Pronczuk, Adam Rasgon, Motoko Rich, Amanda Rosa, Dana Rubinstein, Nelson D. Schwartz, Dionne Searcey, Ed Shanahan, Eliza Shapiro, Mitch Smith, Jim Tankersley, Sabrina Tavernise, Hisako Ueno, David Waldstein, Caryn A. Wilson, Edward Wong, Sameer Yasir and Karen Zraick.