WASHINGTON — The Supreme Court on Tuesday allowed the Trump administration to halt the 2020 census count ahead of schedule, effectively shutting down what has been the most contentious and litigated census in memory and setting the stage for a bitter fight over how to use its numbers for the apportionment of the next Congress.

The brief unsigned order formally only pauses the population count while the administration and a host of groups advocating a more accurate census battle in a federal appeals court over whether the count could be stopped early.

As a practical matter, however, it almost certainly ensures an early end because the census — one of the largest government activities, involving hundreds of thousands of workers — cannot be easily restarted and little time remains before its current deadline at the end of this month. In fact, some census workers say, the bureau had already begun shutting down some parts of its count despite a court order to continue it.

The census has been buffeted both by the coronavirus pandemic and the involvement of the Trump administration in what has traditionally been a rigorously nonpartisan, data-driven exercise. Its early end could mean that White House officials, rather than Census Bureau experts, may use the population numbers to determine representation in the House of Representatives and in state and local governments.

President Trump has insisted those numbers should not include undocumented immigrants living in the United States. That conflicts with the mandate of the Constitution that the census count all residents of the country and would almost certainly give more representation to Republicans.

The court’s order gave no reasons, which is typical when the court acts on emergency applications. It said the count could stop while appeals moved forward.

Justice Sonia Sotomayor dissented, saying that “the harms associated with an inaccurate census are avoidable and intolerable.”

ImageThe Supreme Court’s decision on Tuesday effectively halted what has been the most contentious and litigated census count in memory. 
Credit…Anna Moneymaker for The New York Times

The order was a major victory for the Trump administration, which had been rebuffed by both district and appeals courts in its effort to end the count early. It was a bitter defeat for state and local governments and advocacy groups that had sued to keep the population count going despite the administration’s determination to shut it down.

“The court’s action will cause irreversible damage to efforts to achieve a fair and accurate census,” said Kristen Clarke, the president of the Lawyers’ Committee for Civil Rights Under Law, which represents many of the parties that sued. “It’s incredibly disappointing.”

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Terri Ann Lowenthal, a longtime census expert and consultant to several groups pressing for an accurate tally, charged that the disarray caused by the administration’s handling of the count “inevitably will undermine whatever public confidence remains in the census results.”

The administration “could do the right thing, and allow those operations to wind down in an organized way over the next two weeks, or it could continue to push for rushed results, accuracy and quality be damned,” she said. “The commerce secretary’s next steps will tell us everything we need to know.”

Cary Coglianese, a University of Pennsylvania professor who is one of the country’s top experts on administrative law, said the ruling was in some ways unsurprising. “Whatever one makes of the policy impacts underlying the Trump administration’s position on the timetable for finishing the census,” he said, “today’s ruling does generally fit with the court’s longstanding deferential posture” toward actions by federal agencies.

The justices’ decision is but the latest turnabout in a saga that has seen the deadline for completing the population count — originally set for August, after a decade of preparation — shifted to Oct. 31, then abruptly changed to Sept. 30, then restored by courts to Oct. 31.

The legal battle has focused on whether the Commerce Department, which oversees the Census Bureau, had followed federal law when it set the Sept. 30 deadline. But the subtext has always been whether a curtailed count would be accurate enough to set the baseline for allotting political power and trillions of dollars over the next decade — and whether Mr. Trump was seeking to take control of the count for political advantage.

Most experts said a shortened census would only worsen existing undercounts of the people who have always been hardest for census workers to reach — minorities who are suspicious of the government, and the poor and young people, who move frequently and are more difficult to track down.

Because those groups live predominantly in urban areas, an undercount also would be likely to dilute the political power of Democrats who disproportionately represent those areas. For the same reason, Mr. Trump’s plan to exclude unauthorized immigrants from population totals used to allot political power would probably further dilute Democratic representation in many — though hardly all — areas with large immigrant populations.

The deadline for completing the count was originally moved from August to Oct. 31 after the pandemic all but shuttered many census operations last spring. Because of that delay, the Census Bureau said it would move the dates for delivering population figures used by the House and states for reapportionment and redistricting to next April and beyond.

But in August, Commerce Secretary Wilbur Ross ordered the Oct. 31 deadline moved up a month to Sept. 30, saying the extra time was needed to deliver preliminary population totals to Mr. Trump by the statutory deadline of Dec. 31.

Mr. Ross’s action came over the objection of career census experts who argued that it would undermine, perhaps fatally, the accuracy of the count. It forced the bureau to abandon on short notice some of the counting safeguards it was using to make the tally reliable. And it quickly triggered a lawsuit seeking to keep the tally going.

The move came not long after the announcement in July that the administration would seek to exclude undocumented immigrants from the population totals it would send to Congress for reapportioning seats in the House.


Credit…Christian Monterrosa for The New York Times

The two developments appear related. The old deadlines would have allowed the winner of the presidential election to transmit the population totals next spring. Readopting the December deadline would ensure that Mr. Trump controls that process, even if he loses the November election.

The National Urban League, the League of Women Voters, other groups and local governments sued, saying the rushed schedule would undermine the accuracy of the census and “facilitate another illegal act: suppressing the political power of communities of color by excluding undocumented people from the final apportionment count.”

Judge Lucy H. Koh, of the U.S. District Court in the Northern District of California, ordered the bureau to keep working through the Oct. 31 deadline. “Because the decennial census is at issue here, an inaccurate count would not be remedied for another decade,” she wrote. She also suspended the Dec. 31 statutory deadline for submitting the results.

A three-judge panel of the U.S. Court of Appeals for the Ninth Circuit, in San Francisco, turned down a request from the Trump administration to stay Judge Koh’s order and allow it to stop counting before the end of the month. But the panel said it would not bar the administration from trying to comply with the Dec. 31 reporting deadline.

The administration asked the Supreme Court to intervene, saying that only by shutting down field work now could the bureau meet the Dec. 31 deadline, which is set by statute. “The district court’s order constitutes an unprecedented intrusion into the executive’s ability to conduct the census according to Congress’s direction,” Jeffrey B. Wall, the acting solicitor general, told the justices in a brief filed on Wednesday.

“As the law stands,” Mr. Wall wrote, “assessing any trade-off between speed and accuracy is a job for Congress, which set the Dec. 31 deadline and has not extended it, and the agencies, which acted reasonably in complying with that deadline.”

On Saturday, Mr. Wall wrote that as of Oct. 9 the bureau had counted over 99 percent of households in 49 states. But that was widely questioned by census and demographic experts, who cast the number as a public relations estimate that concealed broad gaps in the accuracy of the tally.

Governments with huge financial and political stakes in a full count of their jurisdictions were especially critical of the court’s ruling. The director of New York City’s efforts to increase census turnout, Julie Menin, said the count had “been stolen by the Trump administration, which has interfered at every step of the way.”

Ditas Katague, who has led a nearly $200 million campaign by California to encourage a complete tally, said the decision “could have grave consequences for the next decade” and said the state would continue to fight in court for fair representation during the apportionment of House seats.

While the court’s ruling effectively stops the census, it does not end the legal battle over it. Lawyers for the plaintiffs in the lawsuit said they would turn to making sure that the Census Bureau was not forced to make further cuts in accuracy checks during the monthslong period in which population data is processed and verified.