NEW YORK >> A federal judge blocked the Trump administration today from asking about citizenship status on the 2020 census, the first major ruling in cases contending officials ramrodded the question through for Republican political purposes to intentionally undercount immigrants.

In a 277-page decision that won’t be the final word on the issue, Judge Jesse M. Furman ruled that while such a question would be constitutional, Commerce Secretary Wilbur Ross acted in an “arbitrary and capricious” manner and violated the law.

“He 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 in a decision that squarely laid the blame on Ross.

Ross’ explanations for his decision were “unsupported by, or even counter to, the evidence before the agency,” the judge said.

Among other things, the judge said, Ross didn’t follow a law requiring Congress be given three years’ notice of plans to add a citizenship question to the census.

“We are disappointed and are still reviewing the ruling,” Justice Department spokeswoman Kelly Laco said in a statement. Ross has said the move was not politically motivated.

The ruling came in cases in which 18 states, the District of Columbia, 15 big cities or counties, and immigrants’ rights groups argued that the Commerce Department, which designs the census, failed to properly analyze the effect that the question would have on households with immigrants.

Furman, citing Census Bureau estimates, concluded the citizenship question would depress responses in households with noncitizens by at least 5.8 percent and likely more.

Thus, he said, several states would lose at least one congressional seat based on 2020 census data. Furman said Texas, Arizona, Florida and plaintiffs New York and Illinois face a substantial risk of a seat loss. He said Colorado will suffer a census undercount of at least 0.7 percent.

Adding the question would cause New York, New Jersey, California, Texas, Florida, Nevada, Hawaii, Illinois, Massachusetts, Maryland, Washington, Oregon, New Mexico and the District of Columbia to lose funding too, he said.

Connecticut, Delaware, Iowa, Rhode Island and Minnesota would also suffer, Furman added.

A trial on a separate suit on the same issue, filed by the state of California, is under way in San Francisco. The U.S. Supreme Court is poised to hear evidentiary-related legal issues surrounding the New York case on Feb. 19.

In the New York case, the plaintiffs accused the administration of President Donald Trump of adding the question to intentionally discourage immigrants from participating, potentially leading to a population undercount — and possibly fewer seats in Congress — in places that tend to vote Democratic.

Even people in the U.S. legally, they said, might dodge the census questionnaire out of fears they could be targeted by the administration.

The Justice Department argued Ross had no such motive.

Ross’ decision to reinstate a citizenship question for the first time since 1950 was reasonable because the government has asked a citizenship question for most of the past 200 years, Laco said.

When Ross announced the plan in March, he said the question was necessary to help the government enforce the Voting Rights Act, a 1965 law meant to protect political representation of minority groups.

Furman, appointed to the bench by former President Barack Obama, said Ross’ rationale concealed his true reason, which remains unknown.

New York Attorney General Letitia James, whose office was among those litigating the lawsuits, called the decision a win for “Americans who believe in a fair and accurate count of the residents of our nation.”

In Massachusetts, Secretary of the Commonwealth William Galvin, a Democrat, said “attempting to frighten immigrant communities into not responding was a clear and deliberate effort to depress the count in states like Massachusetts.”

Ross said politics played no role in the decision, initially testifying under oath that he hadn’t spoken to anyone in the White House on the subject.

Later, however, Justice Department lawyers submitted papers saying Ross remembered speaking in spring 2017 about it with former senior White House adviser Steve Bannon and then-Attorney General Jeff Sessions.

The Supreme Court blocked Ross from being deposed, but let a trial proceed over the objections of Justices Samuel Alito, Clarence Thomas and Neil Gorsuch.

The constitutionally mandated census is supposed to count all people living in the U.S., including noncitizens and immigrants living in the country illegally.

The administration faces an early summer deadline for finalizing questions so questionnaires can be printed.