WASHINGTON — The White House has begun laying the groundwork for a declaration of national emergency to build President Trump’s border wall, a move certain to set off a firestorm of opposition in Congress and the courts but one that could pave the way for an end to the three-week government shutdown.

The administration is eyeing unused money in the Army Corps of Engineers budget, specifically a disaster spending bill passed by Congress last year that includes $13.9 billion that has been allocated but not actually spent for civil works projects, two people with knowledge of the developments said Thursday.

Trump has urged the Army Corps to determine how fast contracts could be signed and whether construction could begin within 45 days, according to one of the people who spoke on the condition of anonymity to describe the preparations.

The list includes dozens of flood control projects in areas affected by recent natural disasters, including the Texas coastline inundated by Hurricane Harvey and parts of Puerto Rico battered by Hurricane Maria. The military construction budget is also being eyed as a potential source for unspent funds, with billions more potentially available there.

The preparations are taking place with talks at an impasse over Trump’s demands for $5.7 billion to construct more than 200 miles of wall along the U.S.-Mexico border. Democrats are staunchly opposed, leading to a partial government shutdown that on Saturday will become the longest ever in U.S. history.

Some 800,000 federal workers are about to miss their first paycheck since the shutdown began Dec. 22, and problems plaguing shuttered national parks, food inspection processes and other federal services are multiplying.

The Senate unanimously passed legislation Thursday that would guarantee back pay to furloughed federal workers once the shutdown ends, although thousands of government contractors who have been furloughed may never recoup their losses.

Trump, who walked out of a White House negotiating session on Wednesday after House Speaker Nancy Pelosi, D-Calif., refused to agree to pay for his wall, reiterated Thursday that he may well declare a national emergency if Democrats don’t give him what he wants.

“Now if we don’t make a deal with Congress, most likely I will do that,” Trump said to Fox News host Sean Hannity about an emergency declaration in an interview that aired Thursday night. “I would actually say I would. I can’t imagine any reason why not because I’m allowed to do it. The law is 100 percent on my side.”

The president and members of his administration have been depicting a humanitarian and public safety crisis at the border, focusing on drugs flowing into the United States and violence by unauthorized immigrants. There was a significant uptick in border apprehensions in 2018, according to data from U.S. Customs and Border Protection, especially of immigrant families, but border apprehensions remain much lower than the high levels seen in the 1980s through the 2000s.

Asked about a timetable for a national emergency declaration, the president said he would see how it goes with Congress.

But on Capitol Hill there were no signs of progress, and instead lawmakers of both parties were bracing for Trump to declare a national emergency. Democrats were exploring their options on how to respond.

Democratic staffers from leadership offices and relevant committees met Thursday afternoon to discuss a potential response. According to an attendee, the meeting focused on undercutting any case that the border situation constituted a national emergency under the legal definition, and highlighting projects that might be put at risk if Trump were to raid other accounts to fund the wall.

House Democratic leadership staff has explored the possibility of a lawsuit against the administration. But while no final determinations have been made, the current thinking is that Congress likely would not have standing to sue, according to a leadership aide.

State attorneys general or people directly affected by a border wall – such as landowners who own property along the U.S.-Mexico boundary – would likely have to file the lawsuit, and the House could file an amicus brief.

Pelosi declined to say how the House would respond to a national emergency declaration, when questioned at a news conference Thursday.

“If and when the president does that, you’ll find out how we will react,” Pelosi said. “But I think the president will have problems on his own side of the aisle for exploiting the situation in a way that enhances his power.”

Indeed, a number of Republicans have expressed qualms or outright opposition about Trump declaring a national emergency, including members of the House Armed Services Committee who object to the prospect of the administration targeting funds within the Pentagon’s military construction budget.

Others cautioned against the administration taking executive action on an issue that should be Congress’s purview.

“It’s not the way to do it. I can understand why they’re looking at it,” said Rep. Mike Simpson, R-Idaho. “I don’t like the idea of pulling money out of defense and military construction and the Army Corps of Engineers. That’s not a good option.”

Asked Thursday whether she would support Trump invoking national security powers to start wall construction, Sen. Lisa Murkowski, R-Alaska, an Appropriations Committee member in the Senate, replied: “No.”

Dan Eberhart, a Republican donor who is often supportive of Trump, said, “Weaponizing a national emergency to achieve a policy objective is usually something that happens in banana republics, not George Washington’s republic.”

But other Republicans were ready for Trump to take the step.

In a statement Thursday, Sen. Lindsey Graham, R-S.C., accused Pelosi of intransigence that’s brought talks to an end, and said that”it is time for President Trump to use emergency powers to fund the construction of a border wall/barrier. ”

“I hope it works,” Graham added.

“There’s no question, it’s perfectly legal,” said Rep. Doug Collins, R-Ga. “I wish we didn’t have to.”

While most Democrats said Trump would be acting recklessly and illegally if he declared a national emergency, even some Democrats were open to the approach.

“Honestly I would be glad, because then it would get shut down in court and we could move on,” Rep. Katie Hill, D-Calif., a freshman who unseated a Republican in a swing suburban district. “Hopefully he figures that out pretty quick.”

One Democratic aide called an emergency declaration an “elegant way out of this mess” – one that would allow Trump and Republicans to declare to their most fervent supporters that they had taken Democrats to the brink, while Democrats would quickly move to tie up any construction in the courts.

The House and Senate could move quickly to pass a bill to reopen the government, predicated on assurances from Trump that he would sign the legislation.

However, conservative Rep. Mark Meadows, R-N.C., who talks frequently with Trump, cautioned that declaration of a national emergency would not necessarily lead to reopening the government.

Many Democrats also believe that an emergency declaration would benefit them politically by unifying their party while splitting Republicans, creating unease among some conservatives who have expressed discomfort with a president sidestepping the Congress in a way they might see as similar to how former president Barack Obama circumvented Congress on immigration.

The president has various powers to act unilaterally, some claimed as inherent in the Constitution, others specifically delegated by Congress. On Capitol Hill, most lawmakers and aides are anticipating a declaration under the 1976 National Emergencies Act, which set out a formal process for declaring an emergency – and for Congress revoking it.

To override an emergency declaration, both houses of Congress would have to pass a resolution doing so and present it to Trump for his signature – one he would presumably veto.

The administration can expect a flood of court challenges if it proposes to build a wall without explicit congressional authorization. Indeed, a number of organizations are already preparing for litigation, just waiting to see exactly what the president does.

“The use of emergency powers to build a wall is unlawful and we are prepared to sue as needed,” said Lee Gelernt, deputy director of the ACLU Immigrants’ Rights Project, which has helped obtain dozens of court orders blocking Trump administration immigration policies.

“There’s going to be a lot of lawsuits,” said Brian Segee, senior attorney for the Center for Biological Diversity. “We are preparing” for possible litigation now, he said.

Even as the discussions over a national emergency declaration were taking place, a final glimmer of hope for a way out of the impasse was extinguished when Graham declared talks over among a small group of Republican senators who’d been meeting to discuss some kind of broader deal to end the shutdown.

These deal-minded Senate Republicans had shuttled Thursday morning between meetings with Senate Majority Leader Mitch McConnell, R-Ky., and Vice President Mike Pence, batting around a proposal that would include Trump’s desired $5.7 billion in wall funding, and a renewable, three-year status for certain immigrants brought illegally to the country as children, along with other provisions.

But by mid-afternoon Thursday, Pence poured cold water on the idea, telling reporters at the Capitol that Trump wanted to wait on trying to make a deal for so-called dreamers until the Supreme Court had ruled on the Deferred Action for Childhood Arrivals program, the Obama-era program that granted protections to these immigrants.

Graham was glum afterward about where things stood, saying he’d “never been more depressed about moving forward than right now,” and not long after that he issued his statement in support of a national emergency declaration.

At the same time, House Democrats pressed forward with their strategy of passing individual spending bills to reopen portions of the federal government that have been closed in the shutdown.

The House on Thursday passed two more spending bills that would open parts of the government that have nothing to do with border security, largely with Democratic votes. A handful of Republicans joined Democrats in supporting those bills – 12 for a bill funding the departments of Transportation and Housing and Urban Development, and 10 for a bill funding the Agriculture Department, the Food and Drug Administration and other agencies.

But Trump has made clear he would veto these bills, and McConnell has said repeatedly that he will not bring up any legislation that doesn’t have Trump’s support.

“There’s no wall, there’s no deal” Pence told reporters on Capitol Hill.