(Adds commentary from experts, detail on next procedural steps)
By Nick Brown and Jonathan Stempel
NEW YORK, Jan 15 (Reuters) - A federal judge on Tuesday invalidated the Trump administration's addition of a U.S. citizenship question to the 2020 census, the first ruling in a handful of lawsuits that claim the query will hurt immigrants.
U.S. District Judge Jesse Furman in Manhattan said Commerce Secretary Wilbur Ross concealed his true motives in adding the question last March.
Ross had said the question - which has not appeared on the decennial census since 1950 - was necessary to enforce federal laws protecting eligible voters.
Furman's decision will almost certainly be appealed, and could wind up before the Supreme Court this year.
The plaintiffs - 18 U.S. states, 15 cities and various civil rights groups - said that asking census respondents whether they are U.S. citizens will frighten immigrants and Latinos into abstaining from the count.
That could cost their mostly Democratic-leaning communities representation in the U.S. House of Representatives, as well as their share of some $800 billion a year in federal funding.
The plaintiffs alleged that was Ross' plan all along, while he insisted the government needed citizenship data to better enforce the Voting Rights Act, which protects eligible voters from discrimination. Only American citizens can vote in federal elections.
Dale Ho, an attorney with the American Civil Liberties Union who argued the plaintiffs' case, called Furman's ruling "a forceful rebuke of the Trump administration's attempt to weaponize the census."
Kelly Laco, a Justice Department spokeswoman, said the administration was "disappointed," adding that the "government is legally entitled to include a citizenship question on the census, and people in the United States have a legal obligation to answer."
In a 277-page opinion, Furman called Ross' Voting Rights Act rationale "pretextual."
"He announced his decision in a manner that concealed its true basis rather than explaining it," Furman said.
Ross said he added the question at the request of the Justice Department, but evidence at trial showed he independently pushed for it much earlier.
Ross also chose not to heed recommendations from experts - including from within the Census Bureau itself - who said adding the question would lead to an undercount and hurt data quality.
During a two-week trial in November, Justice Department lawyers argued Ross need not reveal every motivating factor, as long as his stated rationale was sound.
They also said he was under no obligation to take advice from experts. Ross "doesn't have to choose the best option" as long as he considers all evidence in good faith, Justice Department lawyer Brett Shumate said at trial.
But Furman said Ross and his aides behaved "like people with something to hide," leading to the "inescapable" conclusion that they "did have something to hide."
The ruling means the Trump administration will have to keep litigating if it wants to preserve what has become one of the most controversial of its hawkish immigration policies.
Furman's ruling bars Ross from re-adding the question unless he "cures the legal defects" in his rationale.
The case is thought likely to wind up before the Supreme Court, but time is short: the Census Bureau must print census forms sometime this spring.
Congress could also ban the question, which would eliminate the need for litigation. Democratic lawmakers have said they plan to use their newfound majority in the House to investigate the question.
Census consultant Terri Ann Lowenthal, a former staff director of the House census oversight subcommittee, said "lawmakers should overturn Secretary Ross' decision, which the court correctly ruled was made in violation of legal requirements."
Furman's ruling muddies a census that is already among the most complicated and expensive in U.S. history.
The first-ever online census, set for April 2020, has been plagued by delays, missed deadlines and the cancellation of key tests, partly from legacy underfunding from Congress.
At least five other lawsuits seeking to quash the citizenship question remain pending.
Some advocates worry it may be too late to assuage the fears of immigrants in a climate of heightened immigration rhetoric.
"Hopefully a lot of damage hasn't already been done, and immigrant communities ... will still come to the door when the census takers come," said Doug Rand, a former Obama White House official who worked on immigration issues. (Reporting by Nick Brown and Jonathan Stempel in New York; additional reporting by Mica Rosenberg Editing by Chizu Nomiyama and Jonathan Oatis)
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