Donald Trump on Tuesday (July 21) signed an order that could block undocumented immigrants from being counted when congressional districts are redrawn next year, Reuters reports. Democrats and civil rights advocates immediately vowed to take the Trump administration to court over the legally questionable move.

Redistricting, which means voting districts are redrawn to reflect changes in the population, is scheduled to happen every 10 years, with the next one happening in 2021, Reuters explains. It will take place after the results of the 2020 U.S. census are in. Reuters reports:

If enacted, the plan could benefit Trump’s Republican Party by eliminating the largely non-white population of migrants in the United States illegally, creating voting districts that skew more Caucasian.

It could also cause populous states with large immigrant contingents to lose seats in the 435-member U.S. House of Representatives, including big left-leaning states like California – currently with 53 seats – and New York, with 27.

The process of drawing voting maps for federal congressional districts is known as apportionment.

Trump said in his order, “Including these illegal aliens in the population of the state for the purpose of apportionment could result in the allocation of two or three more congressional seats than would otherwise be allocated.”

Dale Ho, director of the Voting Rights Project for the American Civil Liberties Union (ACLU), released a statement obtained by The Texas Tribune where he vowed litigation.

“The Constitution requires that everyone in the U.S. be counted in the census,” he said. “President Trump can’t pick and choose. He tried to add a citizenship question to the census and lost in the Supreme Court. His latest attempt to weaponize the census for an attack on immigrant communities will be found unconstitutional. We’ll see him in court, and win, again.”

As Reuters points out, the U.S. Constitution “explicitly says congressional districts must be based on ‘the whole number of persons’ in each district, as counted in each decennial U.S. census.” 

U.S. House of Representatives Speaker Nancy Pelosi (D-Calif.) said in a statement obtained by Reuters that Trump’s memo was “unlawful” and was “designed to again inject fear and distrust into vulnerable and traditionally undercounted communities while sowing chaos with the census.”

New York Attorney General Letitia James, who was a leader in successfully challenging Trump’s attempt to add a citizenship question to the census, threw her own threat Trump’s way. “We will continue to lead this fight because we will not allow the Trump Administration’s anti-immigrant policies to tip the balance of power in the nation,” she said in a statement. 

At this point, it is unclear how the Trump administration would even “gather the data necessary to identify—and exclude—people in the United States illegally,” according to Reuters.