Democrats controlling the US House of Representatives have prepared a vote next week to block President Donald Trump from using a national emergency declaration to fund a wall along the US-Mexico border.
The move has accelerated a showdown in Congress that could divide Republicans and lead to Mr Trump’s first veto.
The Democrats introduced a resolution on Friday to block Mr Trump’s declaration, and Speaker Nancy Pelosi said the House would vote on the measure Tuesday.
It is sure to pass, and the Republican-run Senate may adopt it as well.
Mr Trump quickly promised a veto.
“Will I veto it? 100%,” Mr Trump told reporters at the White House.
Any Trump veto would likely be sustained, but the forthcoming battle will test Republican support for the president’s move, which even some of his allies view as a stretch, and a slap at politicians’ control over the power of the federal purse.
“What the president is attempting is an unconstitutional power grab,” said Democrat Joaquin Castro, the sponsor of the resolution, speaking to reporters. “There is no emergency at the border.”
Mr Trump’s declaration of a national emergency gives him access to about 3.6 billion dollars (£2.7 billion) in funding for military construction projects to divert to border fencing.
But the administration is more likely to tap funding from a federal asset forfeiture fund and defence department anti-drug efforts first.
Mr Trump’s edict is also being challenged in the federal courts, where a host of Democratic-led states such as California are among those that have sued to overturn Mr Trump’s order. The House may also join in.
Ms Pelosi said the House measure would “reassert our system of checks and balances”.
For Democrats, the vote is another chance to challenge Mr Trump over funding for a border wall, the issue that was central to the 35-day government shutdown.
It also puts some Republicans from swing districts and states in a difficult spot, as many have expressed misgivings about Mr Trump’s action despite their support for his border security agenda.
Should the House and the Senate initially approve the measure, Congress seems unlikely to muster the two-thirds majorities in each chamber that would be needed later to override a Trump veto.
Ms Pelosi wrote in a letter to her fellow lawmakers that the Republican president’s “decision to go outside the bounds of the law to try to get what he failed to achieve in the constitutional legislative process violates the Constitution and must be terminated”.
“It is institutional. It is constitutional. It is not political,” Pelosi said.
The battle is over an emergency declaration Mr Trump issued to access billions of dollars beyond what Congress has authorised to start erecting border barriers.
Building his proposed wall was the most visible trademark of Mr Trump’s presidential campaign.
Congress last week approved a vast spending bill providing nearly 1.4 billion dollars (£1 billion) to build 55 miles of border barriers in Texas’ Rio Grande Valley while preventing a renewed government shutdown.
That measure represented a rejection of Mr Trump’s demand for 5.7 billion dollars (£4.3 billion) to construct more than 200 miles.