The census hasn't asked the question of each household since 1950, and a federal judge last month stopped the Commerce Department from adding it to the upcoming count. He questioned the motives of Commerce Secretary Wilbur Ross, and said the secretary broke a "veritable smorgasbord" of federal rules by overriding the advice of career officials.
Ross has maintained that the information is important for several reasons, including enforcement of the Voting Rights Act, and that he carefully considered the advantages and disadvantages of adding the question before making his decision.
Those opposed to the question argue the census response rate will likely fall if households are asked whether undocumented immigrants are present, and make less accurate the once-a-decade "actual Enumeration" of the population required by the Constitution.
That could mean fewer members of Congress for states with large immigrant populations, and less money from federal programs.
"The record in these cases provide overwhelming evidence that the administration's goal in adding a citizenship question was to discourage and deter immigrants and communities of color from participating," said Kristen Clarke, president and executive director of the Lawyers' Committee for Civil Rights Under Law, one of the groups that has challenged the decision. "At the end of the day, the census count stands as one of the most critical constitutional functions our federal government performs and this administration has taken extraordinary steps to jeopardize the possibility of achieving a full and fair count."
The Trump administration had asked the court to bypass its normal procedures and accept the case immediately because it needs an answer by the end of June to print census forms and conduct the count on schedule.
Justices will hear the case in late April and review the 227-page opinion handed down by U.S. District Judge Jesse Furman of New York, rather than require it first to go through the U.S. Court of Appeals for the 2nd Circuit.
The Trump administration, as well as the 18 states, local governments and others challenging Ross's decision, told the court that the decision was so important it warranted exceptional treatment.
As New York, the lead challenger, said in its brief to the court:
"The enumeration affects the apportionment of representatives to Congress among the states, the allocation of electors to the electoral college, the division of congressional districts within each state, the apportionment of state and local legislative seats, and the distribution of hundreds of billions of dollars of federal funding."
Solicitor General Noel Francisco told the court that Furman had exceeded his authority.
The district court took the "unprecedented step of striking a demographic question from the decennial census and thereby preventing the Secretary of Commerce from exercising his delegated powers" to decide how the census is conducted, Francisco's brief told the court.
"Indeed, to the government's knowledge, this is the first time the judiciary has ever dictated the contents of the decennial census questionnaire."
But Furman, and the states challenging Ross' decision, said that Congress has placed restrictions on what kind of information the secretary may seek, and the process for implementing it.
Ross "failed to consider several important aspects of the problem; alternately ignored, cherry-picked, or badly misconstrued the evidence in the record before him; acted irrationally both in light of that evidence and his own stated decisional criteria; and failed to justify significant departures from past policies and practices," Furman wrote.
Ross announced the decision to add the question in March. He said at the time that he was responding to a request from the Department of Justice, which said the information was needed to enforce laws protecting minority voting rights.
But later emails and depositions in the lawsuit showed that Ross had discussed the issue with White House officials urging a crackdown on undocumented immigrants. Some showed that he initiated contact with Justice Department officials, not the other way around.
The New York brief says Ross acted directly against the advice of career Census Bureau experts.
"For at least the last forty years, the bureau has vigorously opposed adding any such question based on its concern that doing so 'will inevitably jeopardize the overall accuracy of the population count' by depressing response rates from certain populations, including noncitizens and immigrants," wrote New York Attorney General Letitia James.
The issue already has divided the court to some extent.
The challengers had wanted to depose Ross about his reasons for adding the question, and Furman had agreed. But the administration went to the Supreme Court for a stay of that decision, and the justices said the Cabinet secretary could not be questioned.
Justices Clarence Thomas and Neil Gorsuch would have stopped the trial and not allowed questioning of other officials. Gorsuch said there was no reason to doubt Ross's motives.
The case is Department of Commerce v. New York.