WASHINGTON — A federal judge blocked the Commerce Department from adding a question on American citizenship to the 2020 census, handing a legal victory Tuesday to critics who accused the Trump administration of trying to turn the census into a tool to advance Republican political fortunes.

The ruling marks the opening round in a legal battle with potentially profound ramifications for federal policy and for politics at all levels, one that seems certain to reach the Supreme Court before the printing of census forms begins this summer.

The upcoming census count will determine which states gain or lose seats in the House of Representatives when redistricting begins in 2021. When the Trump administration announced last year it was adding a citizenship question, opponents argued the results would undercount noncitizens and legal immigrants — who tend to live in places that vote Democratic — and shift political power to Republican areas.

In a lengthy and stinging opinion, Judge Jesse Furman of U.S. District Court in New York said that Wilbur Ross, the commerce secretary, broke “a veritable smorgasbord” of federal rules when he ordered the citizenship question added to the census nearly a year ago. Furman said Ross cherry-picked facts, ignored or twisted contrary evidence and hid deliberations from Census Bureau experts.

Furman also criticized Ross and his aides for giving false or misleading statements under oath as they struggled to explain their rationale for adding the question.

The Trump administration argued in court that Ross had the power to add the question and that past surveys had asked about citizenship. On Tuesday, a Justice Department spokeswoman, Kelly Laco, said officials were disappointed by the ruling and were still reviewing it.

“Our government is legally entitled to include a citizenship question on the census and people in the United States have a legal obligation to answer,” she said in a prepared statement. “Reinstating the citizenship question ultimately protects the right to vote and helps ensure free and fair elections for all Americans.”

The Trump administration’s next move is unclear. Government lawyers could appeal the ruling or seek a stay in the 2nd U.S. Circuit Court of Appeals — or go straight to the Supreme Court.

Roughly 24 million noncitizens live in the United States, and nearly 11 million of them illegally. Nearly 1 in 10 households includes at least one noncitizen. Evidence in the eight-day trial in November indicated that the question could deter not just noncitizens and immigrants, but Hispanics and others of foreign descent.

The 14th Amendment requires the House to be apportioned based on “the whole number of persons in each state,” and the Supreme Court has long ruled that the “whole number” includes noncitizens.

“This ruling is a forceful rebuke of the Trump administration’s attempt to weaponize the census for an attack on immigrant communities,” said Dale Ho, director of the ACLU’s Voting Rights Project, adding that evidence at the trial “exposed how adding a citizenship question would wreck the once-in-a-decade count of the nation’s population.”

Ross, whose department oversees the Census Bureau, stated initially that he first examined the need for the citizenship question after getting a request from the Justice Department. In a letter to the Commerce Department, a senior Justice Department official stated that census data on citizenship would help the agency more precisely determine whether the composition of political districts met the mandates of the 1965 Voting Rights Act.

Ross said his review of whether to add the question did not support warnings that it would lead to an undercount of noncitizens and minorities.

Furman, who was nominated to the federal bench by President Barack Obama in 2011, all but demolished that explanation in his ruling.

Ross “materially mischaracterized” a conversation with a polling expert to make it appear that she did not object to adding the question, Furman said, and he kept Census Bureau officials in the dark about his desire for a citizenship question for nearly a year, forgoing any chance for a detailed study of its ramifications. The judge also rejected a sworn deposition on aspects of the question by Ross’ chief aide, Earl Comstock, calling it “misleading, if not false.”

But while Ross’ violations were “egregious,” Furman said, there was not sufficient evidence to prove, as plaintiffs in the lawsuit had claimed, that he had deliberately sought to discriminate against noncitizens and minorities who were most likely to be affected by the citizenship question. In part, he said, that was because the Supreme Court had blocked the plaintiffs from taking sworn testimony from Ross about his actions.

In his ruling, Furman said he could not discern Ross’ real reason for seeking to add the question to the census. But he said it was clear that Ross and his aides had decided within months of taking office in February 2017 that he wanted to add the question, and that the goal of the Justice Department letter “was to launder their request through another agency — that is, to obtain cover for a decision that they had already made.”

Internal documents produced in the lawsuit showed that Ross had discussed the citizenship issue early in his tenure with Steve Bannon, the former White House chief strategist and an architect of the Trump administration’s tough policies against immigrants, and that he had met at Bannon’s direction with Kris Kobach, the former Kansas secretary of state and a far-right opponent of immigration.

Other internal documents showed that Justice Department officials actually had not initiated a request for a citizenship question and in fact had rejected an initial plea from the Commerce Department to do so in the summer of 2017.

In sworn testimony, the department’s senior civil rights official conceded that census data was not necessary to enforce the Voting Rights Act and that citizenship information from the American Community Survey and its predecessor had been used for decades without difficulty.

The Census Bureau itself had recommended against adding a citizenship question, estimating in an analysis last January that at least 630,000 households would refuse to fill out the 2020 questionnaire if such a question were included.

Government lawyers argued repeatedly that none of that mattered, because Ross had broad leeway. That statistical experts disagreed with Ross was “immaterial to this case,” Deputy Assistant Attorney General Brett Shumate told the court. “All the secretary is required to do is to provide a reasoned explanation,” he said. “He doesn’t have to choose the best option.”

Furman said he agreed. But he added that federal law requires that before making such a decision, an agency has to study the relevant information, arrive at a rational decision supported by evidence and honestly articulate its reasons for the decision.

He concluded in his opinion that the record “makes plain that Secretary Ross’ decision fell short on all these fronts.”

This article originally appeared in The New York Times.