U.S. President Donald Trump signed a memorandum on Tuesday that would prevent migrants who are in the United States illegally from being counted when U.S. congressional voting districts are next redrawn, triggering swift rebukes from Democrats and threats of litigation. 

U.S. census experts and lawyers say the action is legally dubious, and not easily executed in practice. Democratic-led states, including New York, and civil rights groups have already vowed to mount a legal challenge or said they are considering it.

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. 

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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 U.S. census are in. 

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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 U.S. legally. 

U.S. House of Representatives Speaker Nancy Pelosi, a Democrat, said in a statement the measure was "unlawful" and was "designed to again inject fear and distrust into vulnerable and traditionally undercounted communities, while sowing chaos with the Census." 

"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 meet litigation.

'Anti-gravity' 

Some see the order as mainly theater – especially because it is unclear how Trump would gather the data necessary to identify – and exclude – people in the United States illegally. 

In 2019, the president signed an executive order calling on states to provide his administration with government records, such as driver's license databases, that could be used to determine the non-citizen population. The order is still facing litigation from immigration advocates. 

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Other surveys conducted by the U.S. Census Bureau provide estimates of the non-citizen population. But that data is incomplete and unreliable, demographers say. 

"There is no count," said Justin Levitt, a professor at Loyola Law School, in Los Angeles, and former U.S. 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." 

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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 census to identify the number of undocumented immigrants have faced roadblocks, most notably in June 2019 when the Supreme Court rejected his attempt to add a citizenship question. 

(With input from agencies)