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Several months ago, a Census Bureau researcher warned that test surveys ahead of the 2020 Census were already showing “unprecedented” levels of concern from immigrants, who fear that answering the survey could lead to deportation, though that would be an illegal use of the data.

On Monday night the Commerce Department announced a change that’s likely to make the problem much worse: in keeping with a request from the Justice Department, the next Census will include a question about citizenship status. Commerce Secretary Wilbur Ross said in a statement that he 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.”

While it might seem like the epitome of boring government bureaucracy, the count of every person living in America — which must take place every ten years, according to the Constitution — is extremely important, as the data is used to draw political districts and determine the distribution of federal funding. The Trump administration’s move is likely to give Republicans, who already have an edge in congressional and state legislative maps, an even greater advantage for years to come.

The citizenship question has not been included in the decennial Census since 1950, though it is included in some smaller population surveys. Adding the question is expected to discourage noncitizens — a population already difficult to count accurately — from responding to the Census. A majority of undocumented immigrants live in 20 metropolitan areas, so undercounting this population would shift power and resources away from more Democratic-leaning cities and toward Republican-leaning rural areas.

As the Washington Post explains, the shift could give Republicans an additional tool in years to come by providing a count of U.S. residents who are eligible to vote. The courts have long mandated that U.S. House districts must be based on the total population, which includes children, undocumented immigrants, and others who cannot vote. However, for many years conservative advocates have discussed the possibility of drawing state and local districts based only on eligible voters — which would exclude nonvoters, who are disproportionately minority.

In 2016 the Supreme Court ruled that states can continue using the total population to draw legislative districts, but left open the possibility of states switching to the count of eligible voters instead. At the time that was only a hypothetical option, as there was no usable Census data on citizens versus noncitizens in a given district.

The Trump administration’s decision changes that. The inclusion of the citizenship question may allow states to draw their legislative maps based on eligible voters in the years ahead. That could indirectly help the GOP maintain a hold on Congress, as the congressional maps would be drawn by more heavily Republican state legislatures.

There’s ample evidence that the Trump administration has been trying to use the 2020 Census to their political advantage, from a foiled attempt to put a leading defender of GOP gerrymandering in charge of the Census, to refusing to hire noncitizen Census-takers, who are seen as crucial to reaching immigrant communities.

Democrats and civil-rights groups have been speaking out against efforts to politicize the Census, and Monday’s decision was met with immediate protest. Xavier Becerra, California’s attorney general, announced that the state intends to sue the Trump administration over the inclusion of the citizenship question. Earlier on Monday Becerra and California secretary of State Alex Padilla wrote in an op-ed that the move is illegal, and would harm their state:

California, with its large immigrant communities, would be disproportionately harmed by depressed participation in the 2020 census. An undercount would threaten at least one of California’s seats in the House of Representatives (and, by extension, an elector in the electoral college). It would deprive California and its cities and counties of their fair share of billions of dollars in federal funds.

Former U.S. attorney general Eric Holder, who has devoted his post-Obama administration career to fighting the GOP’s state-level advantage, said he intends to sue as well.

“We will litigate to stop the Administration from moving forward with this irresponsible decision,” said Holder, who is now chairman of the National Democratic Redistricting Committee. “The addition of a citizenship question to the Census questionnaire is a direct attack on our representative democracy.“