US President Donald Trump threatened on Thursday to declare a national emergency to circumvent Congress if he can't reach a deal with Democrats to fund his promised border wall, which has been at the centre of the ongoing partial government shutdown.

The comments came ahead of his planned trip to Texas to press his case that the country is facing a crisis which can only be solved by spending billions of dollars to construct a wall along the border with Mexico.

His trip to the border town of McAllen, Texas, comes on the 20th day the shutdown that has left some 800,000 Americans out of work or working without pay.

Prior to the shutdown, he said he would be "proud" to shut the government down over the issue but has since blamed Democrats.

He also has been considering whether to declare a national emergency and use it to circumvent Congress by building the wall with money allocated for the Department of Defense.

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On Thursday, Trump said it would be "very surprising" for him not to declare a national emergency if he can't make a deal with Democrats.

"I'm not prepared to do that yet, but if I have to, I will," Trump told reporters. "We're either going to have a win - make a compromise - because I think a compromise is a win for everybody - or I will declare a national emergency," he said.

It's not clear what a compromise would entail, as Trump has so far refused to give up on his demand for $5.7bn in border wall funding, and if he were to go through with the threat to declare a national emergency, it would likely be challenged in the courts.

Democrats, who control the House of Representatives, refuse to approve the funding, saying a border wall is ineffective, expensive and immoral. They have instead said they will allocate more than $1.3bn for border security measures that don't include a wall.

Texas visit

On his trip to Texas, Trump will travel with the state's two US senators, Republicans John Cornyn and Ted Cruz. After Trump's midday visit, Cornyn will host a roundtable discussion with area mayors, judges, law enforcement personnel and others involved with the border issue.

McAllen is located in the Rio Grande Valley, the busiest part of the border where individuals cross between official ports of entry.

Trump has expressed his own doubts that his appearance and remarks in Texas will change any minds as he seeks money for the wall that has been his signature promise since his presidential campaign.

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On December 22, about 25 percent of the government - excluding mainly the Department of Defense and health-related programmes - shut down because of Congress' inability to meet a September deadline on funding.

'Another temper tantrum'

The impasse has continued, while Trump's meetings with Democratic congressional leaders have ended in bitterness. On Wednesday, he stormed out of a meeting with House Speaker Nancy Pelosi and Senate Democratic leader Chuck Schumer, calling it "a total waste of time".

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Trump says undocumented immigrants and illegal drugs are streaming across the border from Mexico, despite statistics that show irregular immigration is at a 20-year low and that many drug shipments likely are smuggled through official ports of entry.

Democrats accuse Trump of using fear tactics and spreading misinformation about the border situation in order to fulfil a 2016 campaign promise as he looks towards his race for re-election in 2020.

"Again, we saw a temper tantrum because he couldn't get his way," Schumer told reporters on Wednesday after Trump walked out of the meeting with Democrats.

The president has been working to make his case to the public and bolster any congressional Republicans who might be wavering.

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Pressure on them could intensify on Friday when hundreds of thousands of federal employees - including border patrol agents and airport security screeners - miss their first paycheques.

On Tuesday, Trump said in his first prime-time television address from the Oval Office that there was a growing security and humanitarian crisis at the border.

On Wednesday, he visited Republican politicians at the US Capitol, emerging from a meeting to say his party was "very unified".

Less than two hours later, eight Republicans in the House voted with majority Democrats on a bill that would reopen the Treasury Department along with some other programmes and did not include any funding for the wall.

Republican Senate Majority Leader Mitch McConnell has made clear, however, that he will not allow that chamber to vote on any measure that does not include wall funding.