President Trump will declare a national emergency to free up funding for border security measures along the southern border, in addition to signing a compromise bill that doesn't include as much funding as he wanted.

Senate Majority Leader Mitch McConnell announced Mr. Trump's intention from the Senate floor Thursday afternoon, which White House press secretary Sarah Sanders quickly confirmed.

"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," Sanders said in a statement. "The president is once again delivering on his promise to build the wall, protect the border, and secure our great country."

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Mr. Trump had long floated the possibility of a national emergency, saying he would probably move forward with one if Congress failed to provide him with enough border wall funding. Sanders said the president will sign a Department of Homeland Security funding bill as well to avoid another government shutdown.

"If this doesn't work out, I probably will do it, I would almost say definitely," the president said of a national emergency declaration when he visited the border in Texas last month, adding, "If we don't make a deal, I would say 100 percent, but I don't want to say 100 percent."

It's still unclear exactly what powers the president will cite to declare some form of national emergency. The White House has released no details about how the president intends to do this, and called a "lid" at 5 p.m., meaning reporters won't see the president for the rest of Thursday.

Congress could look to pass a joint resolution disapproving the president's declaration, but doing so with a veto-proof majority is unlikely.

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An executive action declaring the nation is in a state of a national emergency is sure to meet legal challenges, something Republicans cautioned the president about in recent weeks. Groups announced their intention to file a lawsuit shortly after the White House announcement.

Protect Democracy and the Niskanen Center said in a statement they have prepared a lawsuit and will file it if Mr. Trump goes ahead with his emergency declaration.

When it comes to building border barriers, the Trump administration also faces legal challenges on the eminent domain front. Mr. Trump told reporters last month much of the border security funding has already gone towards purchasing land. The president insists any eminent domain cases, without presenting evidence, would move swiftly.

"What we're doing with eminent domain is in many cases we'll make a deal up front we've already done that," the president told reporters in a Rose Garden press conference in January.

"The secretary has done a lot of that. And if we can't make a deal we take the land and we pay them through a court process. Which goes actually fairly quickly. And we're generous. But we take the land otherwise you could never build anything. If you didn't use eminent domain you wouldn't have one highway in this country. You have to use eminent domain."

Some in Congress, including many Republicans, have suggested a national emergency could be executive overreach. Sen. John Cornyn, R-Texas, told CNN earlier in February that whether presidents have the power to call national emergencies on such issues was a "serious constitutional question."

"The whole idea that presidents — whether it's President Trump, President Warren or President Sanders — can declare an emergency and somehow usurp the separation of powers and get into the business of appropriating money for specific projects without Congress being involved, is a serious constitutional question," Cornyn said.

Sen. Johnny Isakson, R-Georgia, told reporters Tuesday that he hoped Mr. Trump would not call a national emergency, due to "the opportunity it opens for even more mischief which slows us down."

Sen. Susan Collins, R-Maine, also expressed her opposition to calling a national emergency to build the wall, telling reporters that she saw "no grounds" to do so.

"I do not believe that it was intended to apply to a situation where funds have already been allocated for specific purposes by the Congress and then would be repurposed by the President," Collins said on Tuesday. "I think that's dubious from a constitutional perspective and I hope the President will not go that route."

However, other Republicans, particularly staunch Trump ally Sen. Lindsey Graham, have expressed their support for calling one to build the wall.

"If White House and Congress fail to reach a deal then President @realDonaldTrump must act through emergency powers to build wall/barrier," Graham tweeted at the end of January.

Members of the conservative House Freedom Caucus — who railed against President Obama's use of executive action — also urged the president to use executive action in a letter Thursday. On Sunday, House Freedom Caucus Chairman Mark Meadows told CBS News "Face the Nation" host Margaret Brennan Sunday that getting Democrats on board with a legislative solution was preferred, "But until we do that, why should we allow a Democrat president in the White House to use executive orders and- and not do the same with a Republican president?"

Does the president have the authority to declare a national emergency?

"The shorter answer is nobody knows," said Matt Dallek, an associate professor at George Washington University's Graduate School of Political Management who studies presidential powers.

"The reason for that is as with so much else Trump, what's he's doing would really smash through another constitutional guard rail in the sense that he would be explicitly doing an end run around Congress."

The president, Dallek said, has "100 percent" undermined his own argument by stalling in declaring an emergency until now.

"There's no real sort of reality-based reason for declaring a national emergency now," Dallek said.