Donald Trump, facing the prospect of the longest US government shutdown in history, is considering declaring a national emergency that would likely escalate a policy dispute with Democrats over his border wall into a court test of presidential power.
To escape a political trap of his own making, Trump suggested strongly on Thursday that he might declare an emergency so that he can bypass Congress to get funding for his wall, which was a central promise of his 2016 election campaign.
He originally pledged Mexico would pay for the wall, which he says is needed to stem the flow of illegal immigrants and drugs. But the Mexican government has refused. Trump is now demanding that Congress provide $US5.7 billion ($A7.9 billion) in US taxpayer funding for the wall.
That is opposed by Democrats in Congress, who call the wall an ineffective, outdated answer to a complex problem.
The stand-off has left a quarter of the federal government closed down and hundreds of thousands of federal employees staying home on furlough or working for no pay and set to miss their pay cheques.
The partial federal government shutdown entered its 21st day on Friday. It will become the longest shutdown ever on Saturday.
With no Capitol Hill compromise in sight, Trump publicly ruminated on Thursday during a trip to the Texas border about declaring an emergency.
The Wall Street Journal, NBC and the Washington Post, citing unnamed sources, reported that the White House had asked the U.S. Army Corps of Engineers to look into diverting money from its budget toward the wall and to explore how fast construction could begin under an emergency declaration.
Critics of the national emergency strategy have said it may be illegal. In any case, it was almost certain to trigger an immediate court challenge from Democrats.
That would push the wall impasse into the courts, allowing the government to be fully reopened while the judges weigh the case, which could take months.
Partial government funding expired on December 22, leaving departments ranging from Justice, Agriculture and Treasury to Commerce and Homeland Security without money to operate programs and pay their workers.
Senator Joe Manchin, a moderate Democrat who has had good relations with Trump, said declaring a national emergency would be "wrong, but I think that's his only way out."
Manchin predicted that if Trump made the declaration, Congress would immediately move to pass bills funding the various agencies, knowing that the president would then be able to sign them into law.