By Richard Cowan and Steve Holland

WASHINGTON (Reuters) - President Donald Trump vowed on Thursday to declare a national emergency in an attempt to fund his U.S.-Mexico border wall without congressional approval, a step likely to plunge him into a court battle with Congress over constitutional powers.

Conceding defeat in his demand that Congress provide him with $5.7 billion in wall money, Trump agreed to sign a government funding bill that lacks money for his wall but prevents another damaging government shutdown.

The bill, passed by the Republican-led Senate on Thursday, will go to the Democratic-led House of Representatives for final congressional approval. While it contains money for fencing and other forms of border security, it ignores the "great, great wall" that Trump promised in his 2016 campaign to curb illegal immigration and drug trafficking.

"President Trump will sign the government funding bill, and as he has stated before, he will also take other executive action - including a national emergency," White House spokeswoman Sarah Sanders said.

The top Democrat in Congress immediately denounced the president's move. Asked by reporters if she would file a legal challenge to an emergency declaration, House of Representatives Speaker Nancy Pelosi said: "I may, that's an option."

Top Senate Democrat Chuck Schumer accused Trump of a "gross abuse of the power of the presidency."

Republican Senate Majority Leader Mitch McConnell said he would support Trump on the emergency declaration. Earlier this month, McConnell cautioned Trump that declaring an emergency could divide Senate Republicans, the Washington Post reported.

An emergency declaration could infringe on Congress' authority to make major decisions about spending taxpayer funds, a power spelled out as a fundamental check and balance in the U.S. Constitution.

For weeks now, as the president's demands of Congress for wall funding went nowhere, even after a historic 35-day partial government shutdown, the White House has explored the possibility of an emergency declaration that could be invoked to redirect taxpayer funds committed by Congress for other purposes toward paying for Trump's wall.

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'A MISTAKE'

The national emergency strategy, expected to be declared by Trump on Friday, was already dividing Republicans.

Senator Susan Collins, a Republican moderate, said in a statement: "Declaring a national emergency for this purpose would be a mistake on the part of the president."

But Representative Mark Meadows, chairman of a right-wing Republican faction in the House, said: "At this point, President Trump must look at taking executive action - and I will stand with him to protect our sovereign borders."

Congressional aides said House Democrats were expected to file a lawsuit after Trump declares an emergency. The short-term result of that could be a court injunction blocking any diversion of funds while judges weigh the matter, possibly leading in a few months to a Supreme Court decision.

A source familiar with the situation said the White House had identified $2.7 billion in funds previously provided by Congress that could be redirected to barrier funding as part of a national emergency declaration.

The source said lawyers from the White House and other agencies had vetted the figures and believed they would withstand a legal challenge. The source said the money being targeted for diversion represented funds appropriated but not spent, largely from the Defense Department.

SENATE PASSAGE

The Senate passed the government funding legislation by a margin of 83-16. The House was expected to take it up later on Thursday. The measure would provide more than $300 billion to fund the Department of Homeland Security and other agencies through Sept. 30, the end of the current fiscal year.

It includes $1.37 billion in new money to help build 55 miles (88.5 km) of new physical border barriers. That is the same level of funding Congress appropriated for border security measures last year, including barriers, but not concrete walls.

Funding for those agencies is due to expire on Friday, which would trigger another partial federal shutdown on Saturday morning if Congress and Trump failed to act.

Trump triggered the previous shutdown of about a quarter of the federal government with his December demand for $5.7 billion in wall money. He was widely blamed for the shutdown in opinion polls.

In denying funding for Trump's wall, Congress has stood in the way of his ability to deliver on one of his 2016 campaign's key promises and applause lines.

Earlier in the wall fight with Congress, some of Trump's fellow Republicans warned that declaring a national emergency could set a dangerous precedent, opening the door for a future Democratic president to circumvent Congress and declare emergencies on perhaps climate change or healthcare insurance.

Pelosi said: "If the president can declare an emergency on something that he has created as an emergency - an illusion that he wants to convey - just think of what a president with different values can present to the American people." She specifically cited U.S. gun violence as an emergency.

Democratic Senator Ron Wyden said: "I will pull out all the stops to stop this horrendous, bizarre idea. As people get a chance to sort through what the implications really are, we will have strong bipartisan support for opposing this. ... We are the appropriators, not the president."

(Additional reporting David Morgan, Susan Cornwell and Roberta Rampton; Writing by Kevin Drawbaugh and James Oliphant; Editing by Will Dunham and Peter Cooney)