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The US House of Representatives will vote on Tuesday on a resolution aimed at stopping President Donald Trump’s declaration of a national emergency to build a wall on the border with Mexico, Speaker Nancy Pelosi says.
House Democrats introduced the resolution early on Friday, taking the first step to challenge Republican Trump's assertion that he could take money Congress had appropriated for other activities and use it to build the wall.
Pelosi predicted the resolution would pass the Democratic-controlled House. Action would then move to the Republican-majority Senate, where the measure's future is less clear.
In any case, Trump vowed on Friday to veto the measure if it passes both chambers and gets to his desk. Congress would then have to muster the two-thirds majority necessary - a very high hurdle - to override his veto in order for the measure to take effect.
"On the wall? Will I veto it? One hundred percent. One hundred percent, and I don't think it survives a veto. We have too many smart people that want border security, so I can't imagine that it could survive a veto," Trump told reporters in the Oval Office.
About 226 House lawmakers have joined the sponsor, Democratic US Representative Joaquin Castro of Texas, in backing the legislation. The co-sponsors so far include one Republican, Representative Justin Amash of Michigan, Castro said.
"What the president is attempting is an unconstitutional power grab," Castro said in a conference call with reporters. He called on all members of Congress - Democrats and Republicans - to support the resolution terminating Trump’s emergency declaration, saying it tramples on congressional authority and would set a dangerous precedent.
Under the US Constitution, Congress decides how taxpayer dollars are spent. The president can, however, veto spending bills.
Trump declared the national emergency last week after Congress declined to fulfill his request for $5.7 billion this year to help build the wall.
The measure needs only a simple majority in both the House and Senate. It will need the votes of at least four Republicans to pass the Senate, assuming all the Democrats and the two independents there back it.
Pelosi rejected Trump's argument that there is an emergency at the US-Mexico border. "The president of the United States is declaring a national emergency to honor an applause line in a rally," she said.
The issue is also in the courts. A coalition of 16 US states led by California sued Trump and top members of his administration on Monday to block his decision to declare the emergency.
Officials have said that the administration found nearly $US7 billion ($NZ10.2 billion) to reallocate to the wall, including about $3.6 billion from the military construction budget.
A US defence official told reporters Friday said officials had not yet decided what specific programs might be affected, although military housing would not be impacted.
There are about 5000 US active duty and National Guard troops near the border now and that would go up to about 6,000 next month, the official said. He added it could take weeks for an assessment of what Pentagon money might be shifted to the wall project, and months before any construction.
Congress this month appropriated $US1.37 billion for building border barriers following a long battle with Trump, which included a 35-day partial government shutdown - the longest in US history - when agency funding lapsed on December 22.