US President Donald Trump is poised to declare a national emergency at the US-Mexico border, a move that Democrats have vowed to challenge as unconstitutional.
Trump is also expected to sign a bipartisan government spending bill approved by Congress on Thursday that would prevent another federal shutdown by funding several agencies that otherwise would have closed on Saturday morning.
The Republican president was scheduled to deliver remarks on the issue at 10am local time on Friday (2am Saturday AEDT) in the Rose Garden at the White House.
The bill, lacking any money for his wall, is a defeat for Trump in Congress.
His demand for $US5.7 billion ($A8b) in barrier funding yielded no results, other than a record-long 35-day December-January partial government shutdown that damaged the US economy and his poll numbers.
Reorienting his wall-funding quest toward a legally uncertain strategy based on declaring a national emergency could plunge Trump into a lengthy battle with Democrats and divide his fellow Republicans.
Even before the White House said on Thursday that Trump would declare an emergency, Republican senators were sceptical of the declaration that would shift funds to the wall from other commitments set by Congress.
"No crisis justifies violating the Constitution," Republican Senator Marco Rubio said on Twitter on Thursday.
Republican Senator John Cornyn told reporters on Capitol Hill he had concerns about an emergency declaration.
He said it "would not be a practical solution, because there would be a lawsuit filed immediately and the money would be presumably balled up ..."
Some Republicans were more supportive of Trump's tactic.
"I'm not uncomfortable. I think the president's probably on pretty solid ground," Republican Senator Richard Shelby said.
Fifteen Democrats in the Republican-controlled Senate introduced legislation to prevent the transfer of funds from accounts Trump likely would target to pay for his wall.
A senior White House official said the administration had found nearly $US7b ($A9.9b) to reallocate to the wall, including $US600m ($A846m) from a Treasury Department forfeiture fund, $US2.5b($A3.5b) from a Defence Department drug interdiction fund and $US3.5b($A4.9b) from a military construction budget.
The funds would cover just part of the estimated $US23b ($A32b) cost of the wall promised by Trump along the 3200km border with Mexico.
The Senate Democrats' bill also would stop Trump from using appropriated money to acquire lands to build the wall unless specifically authorised by Congress.
Democratic Representative David Price urged lawmakers on the House floor to block Trump's "phony national emergency".
Representative Jerrold Nadler, chairman of the Judiciary Committee in the Democratic-controlled House of Representatives, said he would back a joint resolution to terminate the president's emergency declaration under the National Emergencies Act, and pursue "all other available legal options".
On Thursday evening, the Senate passed the government funding bill by a vote of 83-16, and the House by 300-128, with 86 House Republicans voting in favour.
Trump was expected to sign it and declare an emergency, then fly to his private golf club in Florida for a holiday weekend break.
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