President Donald Trump will sign a border security and federal spending bill to avert another government shutdown but also will declare a national emergency to try to obtain funds for his promised U.S.-Mexico border wall, the White House said on Thursday, drawing immediate criticism from Democrats.

The bipartisan legislation, passed by the Republican-led Senate on Thursday before going to the Democratic-led House of Representatives for final congressional approval, denied Trump the funds he had demanded for a border wall, one of his central 2016 campaign pledges.

In a move that could infringe upon the power of Congress over the national purse strings and plunge Trump into an extended court battle with lawmakers over constitutional powers, White House spokeswoman Sarah Sanders said Trump will declare an emergency. Trump's administration has suggested he could use national emergency powers to redirect money already committed by Congress for other purposes toward paying for his wall.

"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 – to ensure we stop the national security and humanitarian crisis at the border," White House spokeswoman Sarah Sanders said.

But 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." The top Senate Democrat, Chuck Schumer, accused Trump of a "gross abuse of the power of the presidency."

Senate Majority Leader Mitch McConnell, a Republican, said he will support Trump on an emergency declaration.

Congressional aides said House Democrats were expected to file a lawsuit when Trump declares a national emergency in order to protect the House's constitutional powers to decide how U.S. taxpayers' money is appropriated.

In denying funding for Trump's wall, Congress has stood in the way of the president's ability to follow through on one of the key promises and applause lines from his 2016 campaign. Moving to an emergency declaration is a next step that Trump has been exploring for weeks.

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

The source said White House lawyers had vetted the figures and believed they would withstand a legal challenge.

The Senate passed the federal spending 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 U.S. dollars to fund the Department of Homeland Security and a range of other agencies through September 30, the end of the current fiscal year.

The legislation includes 1.37 billion U.S. dollars 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 do not act quickly.

Pelosi accused Trump of doing "an end-run" around Congress and around the Constitution's separation of powers that gives Congress, not the president, federal spending authority.

"It's not an emergency, what's happening at the border. It's a humanitarian challenge to us," Pelosi said.

Earlier in the wall fight with Congress, some of Trump's fellow Republicans had warned him 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."

Trump triggered a 35-day-long shutdown of about a quarter of the federal government with his December demand for 5.7 billion U.S. dollars to help build a portion of the wall. He was widely blamed for the shutdown and agreed to end it without getting the wall money.

The legislation being acted upon on Thursday also funds the Justice Department, Commerce Department, State Department, Department of Agriculture, Internal Revenue Service and others, covering roughly 800,000 federal workers.