California's attorney general filed a lawsuit against the Trump administration over its plans to add a citizenship question to the 2020 U.S. Census, claiming the action is unlawful and "will directly impede" efforts to obtain an accurate population count.

An undercount for California in the upcoming census could result in the nation's most populous state losing a congressional seat and might jeopardize billions of dollars in federal funding for health care, transportation and education, state officials said.

Xavier Becerra, California's attorney general, said in a press conference Tuesday the action was "another reckless decision" by the Trump administration and "threatens not just immigrant families in our state but all of us in the state of California."

The Commerce Department announced late Monday it would reinstate the citizenship question on the 2020 Census to help enforce the 1965 Voting Rights Act. The last time the citizenship question was asked on the Census was in 1950. The decision followed a request by the U.S. Department of Justice, which seeks "to provide census block level citizenship voting age population, or CVAP, data that is not currently available from government surveys," Commerce said in a statement.

"After a thorough review of the legal, program, and policy considerations, as well as numerous discussions with Census Bureau leadership, members of Congress, and interested stakeholders, Secretary [Wilbur] Ross has determined that reinstatement of a citizenship question on the 2020 decennial census questionnaire is necessary to provide complete and accurate census block level data," Commerce said in a statement.



The U.S. Department of Justice said in a statement that the Census Bureau has asked the citizenship question on its American Community Survey since 1965, with the exception of 2010. But it said the ACS "is not the most appropriate data to use as a basis for redistricting. The Justice Department looks forward to defending the reinstatement of the citizenship question, which will allow the department to protect the right to vote and ensure free and fair elections for all Americans."

A Commerce official who declined to be named told CNBC that the department won't comment on specifics of the California lawsuit but insisted the state's case lacks merit. "We look forward to prevailing in court and continuing to work with the Census Bureau to conduct a complete and accurate 2020 census," the official said.