The Washington Post reported earlier this month that McConnell privately cautioned Trump not to use national emergency power to fund the wall because it would face opposition from Republicans.

But the GOP leader changed course on Tuesday when he encouraged Trump to sign an omnibus spending package made up of seven spending bills that provided only a fraction of the $5.7 billion Trump had demanded for the border wall.

"I think he ought to feel free to use whatever tools he can legally use to enhance his effort to secure the border, so no I would not be troubled by that," McConnell told reporters.

His staff later clarified that a national emergency declaration is a legal tool at the president's proposal. But the administration said it's prepared if the courts intervene.

"There is word the president will declare a national emergency. I hope he won't. That would be a very wrong thing to do," he said.

Speaker Nancy Pelosi Nancy PelosiPelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare House lawmakers reach deal to avert shutdown Centrist Democrats 'strongly considering' discharge petition on GOP PPP bill MORE (D-Calif.) separately issued a warning to Republicans, saying that if Trump declares a national emergency to build a border wall that the next Democratic president could do the same to address gun violence.

"A Democratic president can declare emergencies, as well," Pelosi told reporters in the Capitol. "So the precedent that the president is setting here is something that should be met with great unease and dismay by the Republicans."

Pelosi declined to say Thursday if Democrats would launch a legal challenge to Trump's coming emergency declaration, though House Majority Leader Steny Hoyer Steny Hamilton HoyerOn The Money: Powell, Mnuchin stress limits of emergency loans | House seeks to salvage vote on spending bill | Economists tell lawmakers: Kill the virus to heal the economy House moves toward spending vote after bipartisan talks House Democrats mull delay on spending bill vote MORE (D-Md.) suggested the previous day that Democrats would bring such a challenge.

Sanders on Thursday said the White House is ready for any legal challenges from Democrats against a national emergency declaration.

"We're very prepared, but there shouldn't be. The president's doing his job. Congress should do theirs," she told reporters outside her office in the West Wing.

Senate Appropriations Committee Chairman Richard Shelby Richard Craig ShelbySenate GOP eyes early exit Dems discussing government funding bill into February GOP short of votes on Trump's controversial Fed pick MORE (R-Ala.), who spoke to Trump on Wednesday evening, said he supports the national emergency action.

It quickly became apparent on Tuesday and Wednesday that Trump would not have enough money to meet his goal simply from repurposing funds.

Some GOP lawmakers said he could draw on an $800 million defense fund dedicated to drug interdiction but a substantial portion of that money, more than $200 million, was already being used for ongoing anti-drug operations, according to a Republican on the Senate Armed Services Committee who requested anonymity.

Senate Republicans have warned for weeks, however, that an emergency declaration will be subject to a lengthy legal battle and likely will be blocked by a federal judge until adjudicated by an appellate or the Supreme Court.

The emergency declaration is subject to a resolution of disapproval, which must pass both chambers and receive Trump's signature.

House Democrats, who control the lower chamber, are expected to pass the resolution and Schumer can force a vote on the Senate floor because it is privileged.

McConnell has expressed uncertainty over whether he can stop a disapproval resolution from gaining a simple majority to advance to the president's desk but he feels confident he can muster 34 votes to sustain a presidential veto.

"I think there are different opinions about it, and if he goes that route, we'll just hash it out McConnell told reporters on Feb. 5 when asked about the possibility.

"The president could win anyway by vetoing the bill and then trying to get enough votes to sustain it, so [he] may ultimately be able to prevail on the national emergency alternative," he added.

The White House left GOP lawmakers in limbo for hours on Thursday afternoon as the president's lawyers combed through the bill, which spans more than 1,000 pages.

"We are reviewing the bill (which we received at 1am and its over 1000 pages) and as soon as the President makes a decision will let you know," White House press secretary Sarah Huckabee Sanders told The Hill when asked about the holdup.

Acting White House chief of staff Mick Mulvaney Mick MulvaneyMick Mulvaney to start hedge fund Fauci says positive White House task force reports don't always match what he hears on the ground Bottom line MORE on Sunday said the president would take money from whatever sources available to build the border wall that became the flashpoint of a 35-day government shutdown in December in January.

"We'll take as much money as you can give us and then we'll go off and find the money someplace else, legally, in order to secure that Southern [border] but this is going to get built with or without Congress," he said on "Fox News Sunday."

He said "the whole pot is well north of $5.7 billion," referring to the amount Trump requested for the wall.

Less than half, $1.375, will come from the appropriations package Congress is poised to pass Thursday. The rest will come either from using the president's transfer authority to reprogram funding or from the national emergency.

Senate Republicans say whether or not they back the national emergency declaration will depend on how it's structured.

"He's got constitutional powers to do all that," Shelby said when asked about Trump's emergency declaration.

Asked if he would back the move, Shelby said "probably, but I'd have to see what it is."

"I think we have to secure the borders," he added.

Several Republicans immediately expressed concern over Trump’s plan but declined to say how they would vote on a disapproval resolution.

“I’ve expressed my concern about declaring an emergency just in terms of its precedential impacts but also the practical difficulties,” said Sen. John Cornyn John CornynThe Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Liberal super PAC launches ads targeting vulnerable GOP senators over SCOTUS fight Senate GOP faces pivotal moment on pick for Supreme Court MORE (R-Texas). “If there’s a lawsuit and the money gets tied up, it’s not a very practical solution to the problem.”

Sen. Susan Collins Susan Margaret CollinsSenate GOP aims to confirm Trump court pick by Oct. 29: report The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-Maine) said the declaration “will be challenged in court and is of dubious constitutionality.”

“It undermines the role of Congress and the appropriations process and it’s just not good policy,” she added.

Republicans on the Armed Services Committee are raising concerns that the emergency money will likely be pulled away from military construction projects.

"It's a bad idea because it's a bad precedent," said a Senate Republican on the Armed Services Committee. "The most likely or maybe only fund he can allocate from is the [military construction] fund and that hits readiness and a lot of the military installations."

Senate Armed Services Committee Chairman James Inhofe James (Jim) Mountain InhofeChamber of Commerce endorses McSally for reelection Overnight Defense: Top admiral says 'no condition' where US should conduct nuclear test 'at this time' | Intelligence chief says Congress will get some in-person election security briefings Top admiral: 'No condition' where US should conduct nuclear test 'at this time' MORE (R-Okla.) said Trump could build more support for a national emergency declaration if he releases a letter assuring lawmakers that money will not be taken from military construction projects.

– Jordan Fabian contributed

Updated: 5:20 p.m.