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 A.C.L.U.’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.”

Mr. 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 racial or ethnic composition of political districts met the mandates of the 1965 Voting Rights Act.

Mr. 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 who feared disclosing their citizenship status to the government.

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

Mr. Ross “materially mischaracterized” a conversation with a polling expert to make it appear that she did not object to adding the question to the census, Judge 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 Mr. Ross’s chief aide, Earl Comstock, calling it “misleading, if not false.”

But while Mr. Ross’s violations were “egregious,” Judge 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 Mr. Ross about his actions.