WASHINGTON — The Trump administration’s decision to ask people about their citizenship in the 2020 census set off worries among Democrats on Tuesday that immigrants will dodge the survey altogether, diluting political representation for states that tend to vote Democratic and robbing many communities of federal dollars.

Not since 1950 has the census collected citizenship data from the whole population, rather than just a population sample, says the Congressional Research Service. The decision to restore the question after decades prompted an immediate lawsuit from California — already tangling with Washington over immigration — and moves by other states with large immigrant populations to engage in a legal fight.

The population count, a massive effort taken every 10 years, is far more than an academic exercise. It’s required by the Constitution and used to determine the number of seats each state has in the House as well as how federal money is distributed to local communities. Communities and businesses depend on it in deciding where to build schools, hospitals, grocery stores and more.

The political stakes of undercounting segments of the population are high.

Several states that have slowing population growth or high numbers of immigrants such as California, New York, Illinois, Massachusetts and Ohio are typically at risk of losing U.S. House seats when their congressional districts are redrawn every 10 years — depending on how fully their residents are counted.

California struck quickly, with Attorney General Xavier Becerra filling a federal lawsuit Tuesday that seeks to block Commerce Secretary Wilbur Ross’s decision to add a citizenship question in 2020. Officials from New York and New Jersey, also Democratic-led states, were also planning on leading or participating in lawsuits. Massachusetts signaled interest, too.

“The census constitutes the backbone for planning how and where our communities will invest taxpayer dollars,” Becerra said. “California simply has too much to lose to allow the Trump Administration to botch this important decennial obligation.

The Justice Department said in a statement it “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.” The Commerce Department said the benefits of obtaining citizenship information “outweighed the limited potential adverse impacts.”

Their argument in essence: Enforcing voting rights requires more data on the voting-age population of citizens than current surveys are providing.