WASHINGTON (REUTERS) - President Donald Trump signed a memorandum on Tuesday (July 21) that would prevent migrants who are in the United States illegally from being counted when US congressional voting districts are next redrawn, triggering swift rebukes from Democrats and at least one promise of litigation.

US Census experts and lawyers say the action is legally dubious, and not easily executed in practice.

In theory, it 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 US 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.

"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," the memo said.

Redistricting, in which voting districts are redrawn to reflect changes in the population, is next slated for 2021, after the results of the 2020 US Census are in.

Each state will be given a share of the 435 congressional seats based on population. Historically, the distribution of seats has been based on total population, regardless of immigration status. Trump's memo would exclude those not in the US legally.

Tom Wolfe, senior counsel at the left-leaning Brennan Centre, said the memo was "destructive (and) pointless."

'SEE HIM IN COURT'

From a legal standpoint, the move is far from a slam-dunk.

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.

But, while the US Supreme Court has left the door open for citizen-based voting maps for state legislatures, experts see it as a long shot at the federal congressional level.

That is because the US Constitution explicitly says congressional districts must be based on "the whole number of persons" in each district, as counted in each decennial US census.

Dale Ho, an attorney with the American Civil Liberties Union, vowed litigation, saying in a statement, "We'll see him in court, and win," referring to the president.

In the memo, Trump said the word "persons" "has never been understood to include... every individual physically present within a state's boundaries."

In fact it has, census experts say: Multiple federal laws have reinforced that apportionment must include everyone, and US Supreme Court precedent has endorsed that view, said Joshua Geltzer, a constitutional law expert and professor at Georgetown Law.

'ANTI-GRAVITY'

Some see the order as mainly theatre - especially because it is unclear how Trump would gather the data necessary to identify - and exclude - people here illegally.

In 2019, the president signed an executive order calling on states to provide his administration with government records, such as driver's licence databases, that could be used to determine the non-citizen population. Other surveys conducted by the US Census Bureau provide estimates of the non-citizen population.

But those data are incomplete and unreliable, demographers have argued.

"There is no count," said Justin Levitt, a professor at Loyola Law School, in Los Angeles, and former US Department of Justice official under then-President Barack Obama, a Democrat.

"It's as if (Trump) has ordered the National Basketball Association commissioner to implement rules for the use of anti-gravity boots," Levitt said.

"(The commissioner) says anti-gravity boots shall be permitted or shall not be permitted - but they don't exist."

Trump's memorandum could prove popular with the president's base as he tries to generate enthusiasm for his re-election in November, Levitt said.

Trump has spent much of his presidency seeking to limit the number of migrants who illegally enter the United States.

His past efforts to use the US census to identify and limit the political power of undocumented immigrants 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 is still facing litigation from immigration advocates including the Mexican American Legal Defense and Education Fund.