The United States Defense Department sent Congress a request to shift nearly four billion dollars from the military budget to pay for a wall on the border with Mexico, a central promise of President Donald Trump's campaign for the White House four years ago and bid this year for a second term.

Legislators said they received a request on Thursday to reprogramme more than $3.8bn from funding for the National Guard and weapons programmes, setting the stage for a possible confrontation with Democrats.

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Democratic aides said $1.5bn would come from the National Guard, and the rest from funds for procurement, including the Lockheed Martin Corp F-35 fighter jet programme, Lockheed C-130 aircraft, Boeing Co P-8 Poseidon aircraft, Bell Boeing V-22 Osprey helicopters and shipbuilding.

Congressional Democrats, who opposed Trump's past diversion of billions of dollars in military spending to the border wall project, said the decision was dangerous and misguided.

"President Trump is once again disrespecting the separation of powers and endangering our security by raiding military resources to pay for his wasteful border wall," said Democratic Representatives Nita Lowey, chairwoman of the House of Representatives Appropriations Committee, and Pete Visclosky, chairman of the Defense Appropriations subcommittee.

"The Constitution gives Congress, not the President, the power of the purse. Congress rejected President Trump's full request for wall funding, which is why he is now orchestrating this backdoor mechanism to prop up a political vanity project," they said in a statement.

The Trump administration has promised to build at least 640km (400 miles) of wall along the border by November 2020, when Americans will vote for president. In his 2016 campaign, Trump said Mexico would pay for the wall. The Mexican government has consistently refused to do so.

Border Wall System update:



▫️ 119 miles completed

▫️ 220 miles under construction

▫️ 237 miles in pre-construction pic.twitter.com/N57pdvlC8A — Chief Rodney Scott (@USBPChief) February 10, 2020

Trump's hardline immigration policies, particularly for immigrants who come across the southern border with Mexico, have been a signature of his political campaign and first term in the White House, drawing condemnation from rights groups and immigrant advocates.

The Trump administration has built over 160km (100 miles) of border wall, according to US Customs and Border Protection statistics. All of those barriers replaced existing structures, the agency said.