Donald Trump orders US census to exclude illegal immigrants in setting Congress

President Donald Trump ordered the US census on Tuesday to exclude illegal immigrants in its population count for determining representation in Congress, a move that could boost long-term Republican strength.

The unprecedented order challenges the long-accepted constitutional mandate to count every person in the country every 10 years, as the basis for determining representation in the 435-seat House of Representatives.

"For the purpose of the reapportionment of representatives following the 2020 census, it is the policy of the United States to exclude from the apportionment base aliens who are not in a lawful immigration status," Trump's order said.

Excluding undocumented migrants in setting Congressional districts "is more consonant with the principles of representative democracy," the order said.

States that have policies encouraging illegal immigrants "should not be rewarded with greater representation," it said.

It pointed to strongly Democratic California, the country's most populous state with 53 representatives in the House.

The order said that six percent of the state's population are illegal aliens, and if not counted it would lose two or three seats in Congress.

The move brought a sharp backlash from Democrats.

House Speaker Nancy Pelosi, who represents San Francisco in Congress, said the order violates the US constitution and the rule of law and is part of Trump's "cruel anti-immigrant agenda."

"The Constitution is clear: it requires an 'actual Enumeration' of the 'whole numbers of persons' for the population count and congressional apportionment," Pelosi said in a statement.

"Trump's unlawful effort is designed to again inject fear and distrust into vulnerable and traditionally undercounted communities, while sowing chaos with the census," she said.

The American Civil Liberties Union called it "patently unconstitutional."

"The Constitution requires that everyone in the US be counted in the census. President Trump can't pick and choose," said Dale Ho, director of the ACLU's Voting Rights Project.

"We'll see him in court, and win, again," he said, referring to the earlier citizenship question case.

The state of New York, which could be under threat to lose a seat in Congress if Trump follows through, said it would sue to block the move.

"No one ceases to be a person because they lack documentation," New York Attorney General Letitia James said in a statement.

"Under the law, every person residing in the US during the census, regardless of status, must be counted."

The issue of counting non-citizens in the census has been fought over for decades. Legal experts have repeatedly said changing it requires an amendment to the constitution.

Republicans maintain the current counting method favours Democrats, though Trump's party controlled the House for eight years straight from 2011 to 2019 when the Democrats won it back.

The census does have an impact, but not clearly in one party's favour.

In 2010 mostly Republican-leaning states including Texas and Florida gained seats, while losers were split between the parties.

Last year conservative-leaning Alabama sued to have undocumented migrants excluded from the count, saying it would benefit with greater representation in Congress.

The Trump administration last year sought to add a citizenship question to the census count, now well underway, to the same end.

But the Supreme Court ruled that Trump's argument was not constitutionally sound, implying he had merely political motives.

How the census will determine which respondents are legal and which are not is unclear in Trump's order. It tells the secretary of commerce to help determine such information.