Washington: The Democratic-controlled US House of Representatives was expected to easily pass a resolution on Tuesday blocking President Donald Trump's declaration of a national emergency on the border with Mexico, setting up a showdown in the Republican-held Senate.
Trump, after failing to secure funding from Congress for his election campaign promise to build a border wall, declared a national emergency on February 15.
Democrats decried the move as a power grab by the president and a violation of the Constitution because it usurps the power of Congress to decide government spending.
While the measure is expected to sail through the 435-member House, where the Democrats hold a solid majority, its future is uncertain in the Senate, where the Republicans hold a 53-47 seat edge.
At least two Senate Republicans - Senator Susan Collins of Maine and Senator Thom Tillis of North Carolina - have said they would back the resolution overturning Trump's emergency declaration.
Four Republicans would need to break ranks for the measure to pass the Senate but neither chamber is likely to be able to muster the two-thirds majority that would be needed to override an expected presidential veto.
Should the measure clear both the House and the Senate and reach his desk, Trump would be faced with a stinging rebuke and cornered into issuing the first veto of his presidency.
Trump's national emergency declaration gives him access to billions of dollars to construct the border wall beyond the nearly $1.4 billion allocated by Congress.
Democratic House Speaker Nancy Pelosi said Monday that Trump's emergency order was an unconstitutional attempt to expand executive authority and strip lawmakers of the power to control how federal funds are spent.
"The president's power grab usurps that responsibility and fundamentally violates the balance of power envisioned by our founders," Pelosi said.
As several Republican senators expressed reservations about Trump's emergency declaration, the president sought to head them off.
"I hope our great Republican Senators don't get led down the path of weak and ineffective Border Security," Trump tweeted on Monday before heading for Vietnam and a summit with North Korean leader Kim Jong Un.
"Be strong and smart," he said. "Don't fall into the Democrats 'trap' of Open Borders and Crime!"
Republican senators Collins and Tillis have openly broken ranks already, however, and others appeared to be wavering.
While Trump is "rightfully frustrated with Congress's inaction" on border security, the president made a mistake declaring an emergency, said Tillis.
"I have grave concerns when our institution looks the other way at the expense of weakening Congress's power," the Republican senator from North Carolina wrote in The Washington Post.
"The emergency course is not one I favor," Senator Mitt Romney of Utah told reporters.
"I don't think you solve one problem by creating another one, which is taking money out of military construction - and there's separation of powers (issues) as well," added Senator Marco Rubio of Florida. "I don't like it, and my vote will reflect that."
Veto '100 per cent' certain
Trump has said he is "100 per cent" certain to issue a veto if the House and Senate override his emergency declaration, deepening a political showdown on Capitol Hill and setting up a series of legal battles.
With the votes in Congress looming, Trump and the White House have pressed on with plans to repurpose more than $6 billion for wall funding from other sources, mostly already-allocated funds in the Pentagon budget.
Trump claims that rampant illegal immigration has created a "crisis" on the southern border with Mexico, fueling drug smuggling, human trafficking and causing strains on public services.
Two key groups - dozens of Republican former lawmakers, and a bipartisan group of former national security officers including secretaries of defense and CIA directors - spoke out strongly against the emergency declaration on Monday.
In open letters, they warned of an abuse of the framework of the Constitution.
"Under no plausible assessment of the evidence is there a national emergency today that entitles the president to tap into funds appropriated for other purposes to build a wall at the southern border," said the 58 national security officials, who included former secretaries of state Madeleine Albright and John Kerry, and George W. Bush's undersecretary of state Nicholas Burns.