WASHINGTON (Reuters) - President Donald Trump plans to sign a memorandum on Tuesday that aims to prevent migrants who are in the United States illegally from being counted when U.S. congressional voting districts are redrawn in the next round of redistricting

U.S. Census experts and lawyers say the action would be legally dubious, likely benefiting Trump’s Republican Party by eliminating the largely non-white population of migrants in the United States illegally.

Redistricting occurs both at the state level to draw maps for state house legislatures and at the federal level to draw maps for U.S. congressional districts. The latter are known as congressional apportionment.

Proponents of citizens-only voting districts argue each vote should carry the same weight. If one district has far fewer eligible voters than another, they say, each vote there has more influence on election outcomes.

Democrats and immigrant rights activists say including non-voters ensures that elected leaders represent everyone who depends on public services like schools and trash pickup, regardless of voting eligibility.

Trump planned to sign the memo at 12:15 p.m. ET (1615 GMT) in the Oval Office.

White House officials had initially indicated the action would take the form of an executive order, but later indicated it was a memorandum.

A senior administration official said Trump was expected to argue that eliminating illegal immigrants from congressional apportionment maps is necessary to ensure that only legal U.S. residents have a voice in federal politics.

The memo “is another decisive step toward fulfilling his solemn pledge to ensure only American citizens have congressional representation, not illegal aliens,” the official told Reuters, speaking on condition of anonymity.

But the move carries major legal questions, and will probably draw litigation.

Citizens-only voting districts are viewed as potentially legal for state-level voting districts, but the U.S. Constitution explicitly says congressional apportionment should be based on the “whole number of persons” in each state. Multiple federal laws have reinforced that reading, and the U.S. Supreme Court has upheld that interpretation.

“All of this makes Trump’s position outrageous,” said Joshua Geltzer, a constitutional law expert and professor at Georgetown Law, adding that the move will almost certainly be met with litigation.

Another question is how the Trump administration would acquire data on undocumented immigrants. The 2020 U.S. Census does not ask respondents whether they are immigrants, legal or otherwise.

In theory, officials could determine citizenship data through administrative records as drivers’ license databases, along with citizenship estimates gathered in other Census Bureau surveys. But that data is incomplete, and demographers and immigration advocacy groups have argued they are unreliable.

The government’s census count helps in determining where taxpayer money is spent for building public facilities like schools, hospitals and fire departments, as well as calculating states’ apportionment in the U.S. House of Representatives. The U.S. Constitution mandates that the United States count its population every 10 years.

Trump’s planned memorandum could prove popular with the president’s base of support as he tries to generate enthusiasm for his re-election in November.

Trump has spent much of his presidency seeking to limit the number of migrants who illegally enter the United States, particularly from Central America, and his executive order was another part of his immigration agenda.

It has long been a Trump administration strategy to use the census to identify, and limit the political power of, undocumented immigrants. But the efforts have faced roadblocks.

In 2018, the administration said it would ask respondents to the 2020 census whether they were citizens, a move ultimately nixed by the Supreme Court.

Following the defeat, Trump issued an executive order in July 2019 aiming to determine citizenship status through a trove of administrative records. The order, which called on states to turn over such data to the U.S. Census Bureau, is still facing litigation from immigration advocates including the Mexican American Legal Defense and Education Fund.