WASHINGTON — A skeptical Supreme Court on Monday reacted with frustration and some confusion to President Trump’s plan to exclude unauthorized immigrants from the calculations used to allocate seats in the House.
While there was some discussion about whether the plan was lawful, the more immediate questions for the justices were where the administration stood in its efforts to identify and count the unauthorized immigrants and what role the court should play if substantial numbers were not identified.
Removing undocumented immigrants from the census would most likely have the effect of shifting congressional seats and federal money to states that are older, whiter and typically more Republican.
But if the Census Bureau cannot provide Mr. Trump with specific information about a large enough number of unauthorized immigrants in the coming weeks, he will not be able to exclude enough of them from the reapportionment to change the way House seats are allocated. That would leave the justices without a concrete dispute to decide.
“The situation is fairly fluid,” Jeffrey B. Wall, the acting United States solicitor general, told the justices, conceding that the Census Bureau may not be able to supply Mr. Trump with much data on unauthorized immigrants. “There is a real prospect that the numbers will not affect the apportionment,” he said.
Justice Samuel A. Alito Jr. said that put the court in an odd position.
“I find the posture of this case quite frustrating,” he said. “It could be that this is much ado about very little.”
The Commerce Department is required to supply census information to the president by the end of the year, but it may not be able to meet that deadline. Mr. Wall indicated that the Census Bureau had good data on the tens of thousands of people held by Immigration and Customs Enforcement, but the number is almost certainly too small to change apportionment.
He was less certain that the bureau could match records concerning people subject to orders of removal, young immigrants known as Dreamers or other categories of unauthorized immigrants.
If there is data to allow Mr. Trump to exclude enough immigrants to change the allocation of House seats, the case would become quite important.
Indeed, if the court rules for the administration on the core question in the case, it would upend a longtime consensus that the government must count all residents of the United States, whatever their immigration status, with the potential to shift political power and federal money from Democratic states to Republican ones.
But there was only limited discussion of that question on Monday, though several justices, including some conservatives, expressed skepticism about the administration’s arguments.
“A lot of the historical evidence and longstanding practice really cuts against your position,” Justice Amy Coney Barrett told Mr. Wall.
He responded that the novelty of Mr. Trump’s approach did not make it unlawful. “I’ll certainly grant,” Mr. Wall said, “that no president has made this judgment before.”
Most of the argument focused on the practical complications in the case.
Mr. Wall said the process was cumbersome and difficult. “We’re literally trying to individually identify people who are present in the United States in violation of federal law,” he said.
Mr. Wall suggested that the plaintiffs could file a new lawsuit after Mr. Trump submits a statement to Congress setting out the number of representatives to which each state is entitled if House seats shift because of the exclusion of unauthorized immigrants.
Chief Justice John G. Roberts Jr. was skeptical. “Isn’t that going to be like having to unscramble the eggs?” he asked.
Other justices wondered whether the court should simply defer a decision in the current case, which it had put on an exceptionally fast track, until more information becomes available in the coming weeks.
The Constitution requires congressional districts to be apportioned “counting the whole number of persons in each state,” using information from the census. In July, Mr. Trump issued a memorandum taking a new approach. “For the purpose of the reapportionment of representatives following the 2020 census,” the memo said, “it is the policy of the United States to exclude from the apportionment base aliens who are not in a lawful immigration status.”
“Current estimates suggest that one state is home to more than 2.2 million illegal aliens, constituting more than 6 percent of the state’s entire population,” the memo said, apparently referring to California. “Including these illegal aliens in the population of the state for the purpose of apportionment could result in the allocation of two or three more congressional seats than would otherwise be allocated.”
In the memo, Mr. Trump ordered Wilbur Ross, the secretary of commerce, to provide him with two sets of numbers, one including unauthorized immigrants and the other not. But it became unclear how Mr. Ross would derive the second set of numbers after the Supreme Court last year rejected his efforts to add a question on citizenship to the census.
The case before the court, Trump v. New York, No. 20-366, was brought by two sets of plaintiffs, one a group of state and local governments and the United States Conference of Mayors, and the second a coalition of advocacy groups and other nongovernmental organizations.
A three-judge panel of the Federal District Court in Manhattan ruled that the new policy violated federal law. Two other courts have issued similar rulings, while one said the dispute was not ripe for consideration.
In an unsigned opinion in the case from Manhattan, the panel said the question before it was “not particularly close or complicated.”
“The secretary is required to report a single set of figures to the president — namely, ‘the tabulation of total population by states’ under the ‘decennial census’ — and the president is then required to use those same figures to determine apportionment using the method of equal proportions,” the panel wrote, quoting the relevant statutes.
Chief Justice Roberts questioned the first part of the analysis. “It seems to me that you’re asking really for a gag order on the secretary of commerce concerning his communications to the president,” he told Barbara D. Underwood, New York’s solicitor general.
Ms. Underwood said the secretary was free to provide information about unauthorized immigrants in other ways. “It might be usable for many other things,” she said, “but not as part of the apportionment.”
Mr. Wall told the justices that it was lawful to exclude unauthorized immigrants. “It’s the sovereign’s prerogative to define the political community,” he said.
Ms. Underwood responded that the administration was asking the court to endorse a stunning departure from the nation’s traditions.
“The Constitution and laws provide that House seats should be allocated on the basis of total population,” she said. “The framers wanted a system that could not easily be manipulated. So they decided to count just the persons living in each state. The policy here would for the first time in this nation’s history reject that choice.”
Dale Ho, a lawyer with the American Civil Liberties Union, said Mr. Trump’s authority was limited.
“While the president may have some discretion in borderline cases,” Mr. Ho said, “he does not have authority to erase millions of state residents from the apportionment based solely on unlawful immigration status.”
The argument was conducted by telephone, with the justices asking questions one at a time, in order of seniority, subject to strict time limits enforced by the chief justice. At one point, Justice Alito seemed to chafe at that restriction, launching into a six-part question after the chief justice had indicated that his time was up.
He managed to ask three of his six questions before Chief Justice Roberts cut him off.