WASHINGTON — The Supreme Court’s conservative majority seemed ready on Tuesday to allow the Trump administration to add a question on citizenship to the 2020 census, which critics say would undermine its accuracy by discouraging both legal and unauthorized immigrants from filling out the forms.
Justice Sonia Sotomayor said that adding the question would do damage to the fundamental purpose of the census, which is to count everyone in the nation.
“There is no doubt that people will respond less,” she said. “That has been proven in study after study.”
By one government estimate, about 6.5 million people might not be counted.
Solicitor General Noel J. Francisco, representing the Trump administration, acknowledged that the question could depress participation. But he said the information it would yield was valuable.
“You’re always trading off information and accuracy,” he said.
Justices Neil M. Gorsuch and Brett M. Kavanaugh noted that questions about citizenship had been asked on many census forms over the years and were commonplace around the world.
But by the end of the arguments, which lasted 80 minutes instead of the usual hour, the justices seemed divided along the usual lines, suggesting that the conservative majority would allow the question.
Much of the argument concerned statistical modeling. “This gets really, really technical,” Justice Samuel A. Alito Jr. said.
The federal government has long gathered information about citizenship. But since 1950, it has not included a question about it in the census forms sent once a decade to each household. Adding it could reduce Democratic representation when congressional districts are allocated in 2021 and affect how hundreds of billions of dollars in federal spending are distributed.
Courts have found that Arizona, California, Florida, Illinois, New York and Texas could risk losing seats in the House, and that several states could lose federal money.
The case — United States Department of Commerce v. New York, No. 18-966 — is the latest test of the scope of executive power in the Trump era. Last year, the justices upheld President Trump’s authority to restrict travel from several predominantly Muslim countries.
The census case has its roots in the text of the Constitution, which requires an “actual enumeration” every 10 years, with the House of Representatives to be apportioned based on “the whole number of persons in each state.”
In addition to counting the number of people in the nation, the census has also sought other kinds of information. Whatever the Supreme Court rules, the 2020 short form will include questions about sex, age, race and Hispanic or Latino origin. Some of those questions may discourage participation, too.
Wilbur Ross, the commerce secretary, has said he ordered the citizenship question to be added solely in response to a December 2017 request from the Justice Department, which said data about citizenship would help it enforce the Voting Rights Act of 1965.
Three federal trial judges have ruled that the evidence in the record demonstrates that Mr. Ross was not telling the truth. He had long before decided to add the question, the judges found, and he pressured the Justice Department to supply a rationale.
Justice Sotomayor suggested Mr. Ross had manufactured the reason. “This is a solution in search of a problem,” she said.
Documents disclosed in the case showed that Mr. Ross had discussed the citizenship issue early in his tenure with Stephen K. Bannon, the former White House chief strategist and an architect of the Trump administration’s tough policies against immigrants, and that Mr. Ross had met at Mr. Bannon’s direction with Kris Kobach, the former Kansas secretary of state and a vehement opponent of unlawful immigration.
Judge Jesse M. Furman, of the Federal District Court in Manhattan, had called for Mr. Ross to be questioned under oath, but the Supreme Court blocked that order in October. Justice Gorsuch, joined by Justice Clarence Thomas, said the court should have gone further by shutting down all pretrial fact-gathering. Justice Gorsuch added that there was no indication of bad faith in Mr. Ross’s conduct.
“There’s nothing unusual about a new cabinet secretary coming to office inclined to favor a different policy direction, soliciting support from other agencies to bolster his views, disagreeing with staff or cutting through red tape,” Justice Gorsuch wrote at the time. “Of course, some people may disagree with the policy and process. But until now, at least, this much has never been thought enough to justify a claim of bad faith and launch an inquisition into a cabinet secretary’s motives.”
In November, the Supreme Court rejected a request from the Trump administration to halt the trial, over the dissents of Justices Thomas, Gorsuch and Alito.
Douglas N. Letter, representing the House of Representatives, began his argument by conveying thanks from Speaker Nancy Pelosi for allowing him to argue alongside lawyers representing two sets of challengers.
Chief Justice John G. Roberts Jr. smiled. “Tell her she’s welcome,” he said.