Donald Trump has announced he will declare a national emergency to fulfil his pledge to construct a wall along the US-Mexico border.
The president said he will use executive powers to bypass Congress, which approved far less money for his proposed wall than he had sought.
He plans to siphon billions of dollars from federal military construction and counter-drug efforts for the wall.
The move is already drawing bipartisan criticism on Capitol Hill and is expected to face rounds of legal challenges.
“I am going to be signing a national emergency,” Mr Trump said from the Rose Garden at the White House, as he claimed illegal immigration marked “an invasion of our country”.
In a rare show of bipartisanship, legislators voted on Thursday to fund large swaths of the government and avoid a repeat of this winter’s debilitating five-week government shutdown.
The money in the bill for border barriers, about 1.4 billion dollars (£1.1 billion), is far below the 5.7 billion dollars (£4.4 billion) Mr Trump insisted he needed and would finance just a quarter of the 200-plus miles he wanted this year.
To bridge the gap, he announced he will be spending roughly 8 billion dollars (£6.2 billion) on border barriers — combining the money approved by Congress with funding he plans to repurpose through executive actions, including the national emergency.
The money is expected to come from funds targeted for military construction and counter-drug efforts, but aides could not immediately specify which military projects would be affected.
Despite widespread opposition in Congress to proclaiming an emergency, including by some Republicans, Mr Trump was responding to pressure to act unilaterally to soothe his conservative base and avoid appearing like he has lost his wall battle.
Word that he would declare the emergency prompted condemnations from Democrats and threats of lawsuits from states and others who might lose federal money or said the president was abusing his authority.
The top two Democrats in Congress, House speaker Nancy Pelosi and Senate minority leader Chuck Schumer, said they will use “every remedy available” to oppose Mr Trump.
They pledged to take action “in the Congress, in the Courts, and in the public”, adding that Mr Trump’s decision is unlawful and would “shred the constitution” by usurping Congress’s power to control spending.
Democratic state attorney generals said they would consider legal action to block Mr Trump. Puerto Rico governor Ricardo Rossello told the president on Twitter “we’ll see you in court” if he makes the declaration.
Even if his emergency declaration withstands challenge, Mr Trump is still billions of dollars short of his overall funding needed to build the wall as he promised in 2016.
After two years of effort, he has not added any new border mileage. All construction so far has gone in replacing and repairing existing structures. Ground is expected to be broken in south Texas soon on the first new mileage.
The White House said Mr Trump would not try to redirect federal disaster aid to the wall, a proposal they had considered but rejected over fears of a political backlash.