WASHINGTON • US President Donald Trump has signed a memorandum 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 threats of litigation.

US 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 Mr Trump's Republican Party by eliminating the largely non-white population of migrants who are in the US 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 the following year, 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. Mr Trump's memo on Tuesday would exclude those who are not in the country legally.

US House of Representatives Speaker Nancy Pelosi said in a statement that 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".

From a legal standpoint, the move is far from a slam dunk. Proponents of citizens-only voting districts argue that 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.

Mr Dale Ho, an attorney with the American Civil Liberties Union, vowed litigation. Referring to the President, he said in a statement: "We'll see him in court, and win."

New York Attorney-General Letitia James, who helped lead a coalition of states that successfully challenged Mr Trump's attempt to add a citizenship question to the census, made a similar threat.

She said: "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."

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

In fact, it has, census experts say.

Professor Joshua Geltzer, a constitutional law expert from Georgetown Law, said multiple federal laws have reinforced that apportionment must include everyone, and US Supreme Court precedent has endorsed that view.

REUTERS