By Jonathan Stempel
NEW YORK (Reuters) - A group of U.S. states and cities sued the Trump administration to stop it from asking people filling out 2020 census forms whether they are citizens.
The lawsuit by 17 states, Washington D.C. and six cities challenged what they called last week's "unconstitutional and arbitrary" decision by the U.S. Department of Commerce, which oversees the Census Bureau, to add the citizenship question.
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It was also a fresh challenge to what New York Attorney General Eric Schneiderman, at a press conference announcing the lawsuit, called the administration's "anti-immigrant animus."
All of the states bringing the case have Democratic attorneys general.
They were joined by New York City, Chicago, Philadelphia, San Francisco, Seattle and Providence, Rhode Island, which all have Democratic mayors, and the U.S. Conference of Mayors.
Another state, California, filed a similar lawsuit last week.
Asked to comment, a U.S. Department of Justice spokeswoman in an email referred to Attorney General Jeff Sessions' April 2 statement lamenting how California's "meritless" lawsuit forced it to litigate whether the government deserves an "accurate count of who can legally vote in our federal elections."
The U.S. Constitution mandates a decennial census, which is used to determine the drawing of political boundaries, the allocation of seats in Congress and at the state and local level, and the annual distribution of about $700 billion of federal funds.
Critics of the citizenship question say it might dissuade immigrants, and perhaps many citizens, from being counted, with a disproportionate impact on Democratic-leaning states.
Supporters, including Commerce Secretary Wilbur Ross, say the question will help the country enforce the landmark Voting Rights Act of 1965.
A citizenship question has not appeared on the decennial census form since 1950.
The lawsuit accused the Trump Administration of violating the Constitution's requirement for an "actual enumeration" of the "whole number of persons" every 10 years.
At the press conference, Schneiderman called the citizenship question a "blatant effort" by the administration to prevent the Census Bureau from doing its job.
"This is an affront to our national ideals," Schneiderman said. "This is an affront to the Constitution."
The lawsuit said adding the question could particularly exacerbate undercounting of the fast-growing Hispanic population, after an estimated 1.54 percent undercount in 2010.
It said the question would add fuel to a threat made in Congressional testimony last June by Thomas Homan, acting director of U.S. Immigration and Customs Enforcement.
The lawsuit quoted Homan as saying undocumented immigrants "should be uncomfortable. You should look over your shoulder. And you need to be worried."
(Reporting by Jonathan Stempel in New York; Additional reporting by Makini Brice in Washington, D.C.; Editing by Chizu Nomiyama and Richard Chang)