In Florida in 2016, J.C. Planas, a former Republican state representative, was uncomfortable with Hillary Clinton but detested Donald Trump, so he wrote in former Gov. Jeb Bush for president.

In New Hampshire that year, Peter J. Spaulding, a longtime Republican official, supported the Libertarian ticket.

And in Arizona, Lorena Burns, 56, also voted third party, seeing the choice between Mr. Trump and Mrs. Clinton as a contest between “two bads.”

“I didn’t want to be responsible for either,” she said.

This year, all three of them intend to diverge from their Republican leanings and vote for former Vice President Joseph R. Biden Jr., the presumptive Democratic nominee. They are among an emerging group of voters who disliked both major-party presidential nominees in 2016, but who are now so disillusioned with President Trump — and sufficiently comfortable with Mr. Biden — that they are increasingly willing to support the Democrat.

It’s a dynamic that could have significant implications in several of the most competitive battleground states, like Arizona and Wisconsin, where the third-party vote in 2016 was greater than the margin of difference between Mr. Trump and Mrs. Clinton. Recent polling also shows that Mr. Biden has an overwhelming advantage over Mr. Trump among voters who have unfavorable views of both candidates — a cohort that ultimately broke in Mr. Trump’s favor in 2016, exit polls showed.

Ms. Burns of Guadalupe, Ariz., said she recently made her first political donation, to the Democratic National Committee. She said she agreed with many of Mr. Trump’s policies, but was turned off by his behavior. “Just the lying, just the craziness, the bullying — I’d rather pay more money than be with him for another four years,” she said. “I’m willing to pay more money in taxes just to be away from him. He’s corrupting the country.”

In Ms. Burns’s state of Arizona, Mr. Trump won by 3.5 percentage points in 2016. The Libertarian Party nominee, Gary Johnson, won 4.1 percent of the vote, and in other states where the race was even closer — including Pennsylvania, Michigan, Wisconsin and Florida — he pulled in between 2 and 4 percent. The Green Party candidate Jill Stein took in roughly 1 percent in those states — small but significant totals in contests that were decided by slim margins.

In any single poll, it is difficult for pollsters to reach a significant number of voters who supported third-party candidates in 2016, making it impossible to trace their preferences now. And Mr. Trump — who faced vocal opposition that year from some prominent Republicans and won anyway — remains overwhelmingly popular with Republican voters. While many center-right voters have distanced themselves from his party, there are others who initially expressed misgivings about him and have since come to embrace him, resistant to the leftward drift of the Democratic Party.

But in a year when swing voters are scarce, some of the voters who effectively stayed on the sidelines in 2016 are showing signs of political movement now — and there is evidence that Mr. Biden stands to benefit.

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Credit…Patrick Breen/Santa Fe New Mexican, via Associated Press

There appears to be far less interest in third-party candidates compared with the same point in 2016, pollsters say.

“Barring some unforeseen circumstance, there’s just not a lot of appetite for third party,” said Patrick Murray, director of the Monmouth University Polling Institute. “This is two-person for nearly all American voters.”

His polling from late June found that among voters who have unfavorable views of both candidates, Mr. Biden leads the president 55 percent to 21 percent. In 2016, Mr. Trump won the voters who disliked both candidates, according to exit polls.

And according to a recent poll of registered voters in six major battleground states by The Times and Siena College, people who say they did not vote in 2016 overwhelmingly favor Mr. Biden over Mr. Trump in November, 56 percent to 25 percent. Among registered voters in those states who said they did cast ballots in 2016, 47 percent said they planned to support Mr. Biden and 42 percent said they would back Mr. Trump

The Monmouth poll also found that at this point, “fewer voters have a negative opinion of the Democratic nominee” compared with four years ago.

“You had two lightning rod candidates running last time,” said the veteran Republican pollster Glen Bolger, who added that there was still time for Republicans to shape perceptions of Mr. Biden. “At this point in time, Joe Biden isn’t nearly as controversial as Hillary Clinton was, so I think third party candidates are a little bit slower to come out of the woodwork.”

In 2016, voters who went third party spanned the ideological spectrum, from Republicans who did not believe Mr. Trump was a true conservative, to progressives who opposed Mrs. Clinton from the left. The Biden campaign has been working to improve Mr. Biden’s standing with young liberals, aware of the need to engage and mobilize those voters who have long been skeptical of his relatively centrist policy stances. In part because of his difficulty gaining the confidence of young voters and liberals, Mr. Biden’s net favorability rating nationwide remains stuck close to zero.

But Mr. Biden’s team also sees significant opportunities to improve his favorability rating both with disaffected voters who have been moving away from the Democratic Party — voters without college degrees, for example — and with center-right moderates who, in the Trump era, have slipped farther from the Republican Party.

This year, a number of organizations have also mobilized to target Republican-leaning voters who dislike Mr. Trump but do not consider themselves Democrats, aiming to bring them into the Biden fold.

An organization called Republican Voters Against Trump has released testimonials from voters who have never voted for a Democrat before. And veterans of the George W. Bush administration announced a new political action committee last week in support of Mr. Biden; one of the leaders is Kristopher Purcell, who worked in the communications office under Mr. Bush.

Mr. Purcell submitted a write-in vote in 2016. This year, he said, will be the first time he votes for a Democrat for president.

“We have seen over the last four years what a Trump presidency means for the country, and it’s increasingly negative, it’s increasingly damaging,” Mr. Purcell said. “We want to really focus on persuading historically Republican voters.”

Mr. Spaulding of New Hampshire would fall into that category.

He chaired the late Senator John McCain’s presidential bids there in 2000 and 2008, currently serves as a commissioner of Merrimack County in New Hampshire and continues to identify as a Republican. This year, he chaired the New Hampshire campaign of former Gov. William F. Weld, Republican of Massachusetts, who challenged Mr. Trump in the primary before exiting the race in March.

Mr. Spaulding has never voted for a Democrat for president, he said in an interview, but would “probably” support Mr. Biden this time. He called the former vice president “a middle of the roader-type Democrat” who “will do the things that need to be done to get our country back together again.”

Asked if he had any reservations about voting for a Democrat, Mr. Spaulding replied, “not when the stakes are as high as they are this year.”

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Credit…Anna Moneymaker for The New York Times

In interviews, a number of Republican-leaning voters who supported neither major-party candidate last time echoed Mr. Spaulding’s view that they were comfortable with the relatively moderate Mr. Biden, for the very reasons that more progressive voters have been unenthusiastic about his candidacy.

Some people interviewed, however, admitted to being uneasy about whether Mr. Biden would be pushed too far to the left by ascendant voices in the Democratic Party.

“If you look at the entire field the Democratic Party put up, he was probably the most centrist of them all, and it’s that centrist side that leads me to be OK to vote for him,” said Emmanuel Wilder, who voted for Mr. Johnson in 2016 and ran unsuccessfully for the North Carolina Statehouse as a Republican in 2018.

Mr. Wilder intends to vote for Mr. Biden, but added, “I have that concern, whether he will govern like that or whether he will lean more toward following the lead of his party.”

Still, not everyone who opposed the major candidates in 2016 wants to pick a side yet.

“It’s an unbelievably bad choice twice now,” said Richard Vinroot, a Republican former mayor of Charlotte, N.C., who opposed Mr. Trump and Mrs. Clinton in 2016, and will not vote for Mr. Trump or Mr. Biden this year. “I’m very disappointed in the choice that we have.”

The Trump campaign hopes to fuel perceptions that Mr. Biden and the Democratic Party are too radical, seeking to link Mr. Biden to the most progressive voices in his party at a moment of national unrest over racism and policing.

“Our data shows that a lot of people know of Joe Biden, but not very many know much about him,” said Tim Murtaugh a spokesman for Mr. Trump’s re-election campaign, calling Mr. Biden “incapable of standing up to the most extreme elements in his party.”

“By Election Day, voters will be aware of that,” he said.

Yet polling shows that it is Mr. Trump who is out of step with much of the country on issues of racial justice. And Mr. Biden, who has supported protesters of police brutality, has also rejected the most far-reaching measures proposed by some in his party — he opposes defunding the police, for example.

Back in Arizona, Barbara Hill, 85, reflected on her 2016 vote.

“I voted for somebody else on the ballot,’’ she said. “I wasted my vote, in other words, but I couldn’t stand either one of them.”

This time, she said, she will be voting for Mr. Biden.

Nate Cohn contributed reporting.