Congressional Democrats moved rapidly Friday to advance legislation to reject President Trump’s declaration of a national emergency, an effort that will force Republican lawmakers to choose between their support for Trump and their oft-stated opposition to the expansion of presidential power.

The measure, introduced Friday, is expected to pass easily in the Democratic-controlled House, where Speaker Nancy Pelosi (D-Calif.) said she would bring it up for a vote Tuesday. But the GOP’s dilemma will play out in public view days later in the Republican-controlled Senate, where numerous GOP senators have voiced opposition to Trump’s emergency declaration.

If Senate Democrats are united, they will need only four Republican defections to pass the measure and send it to Trump’s desk.

On Friday, Trump said he would veto the measure “100 percent” if that happened. And he predicted that Congress would be unable to muster the votes to override his veto.

President Trump declared a national emergency to build a border wall on Feb. 15. Here's what you missed. (Taylor Turner/The Washington Post)

“We have too many smart people that want border security, so I can’t imagine it can survive a veto,” Trump said, adding that he expects most Republicans to support him. “I think they’ll stick.”

However, conservative commentators and other GOP opinion leaders have been ratcheting up pressure on Republican senators to hold the line against what they view as an unacceptable overreach of presidential authority. For Republicans who routinely accused President Barack Obama of exceeding his constitutional powers, “this should not be a difficult vote,” said Charlie Sykes, a conservative commentator who edits the Bulwark, an online publication critical of Trump.

“There’s not one Republican in Congress who did not object strenuously to Obama’s use of executive authority,” Sykes said. “So it would be monumentally hypocritical for them to turn around and endorse Trump’s use of executive authority to override Congress’s constitutionally mandated power of the purse.”

In a nod to that thinking, Pelosi and other Democrats have couched their arguments against the emergency declaration in constitutional terms, arguing that Congress cannot stand idly by while the president usurps the authority of the legislative branch.

“We have a separation of powers in our country,” Pelosi told reporters Friday. “We battled against a monarchy — we did not intend to establish one in our country.”

While more than a dozen Republican senators have made skeptical comments about Trump’s declaration, only one — Sen. Susan Collins (Maine) — has said she would vote to end it. On Friday, aides to several Republican senators who have been critical of using emergency powers to build the wall — including Sens. Lamar Alexander (Tenn.), Lisa Murkowski (Alaska), Ben Sasse (Neb.) and Thom Tillis (N.C.) — either declined to comment or did not respond when asked if those senators planned to vote against Trump’s emergency declaration.

In the House, Rep. Justin Amash (Mich.) is the only Republican among the more than 220 co-sponsors of the Democratic resolution opposing Trump’s emergency declaration. House Democrats expect at least a few others ultimately to support the measure.

Republican lawmakers have rarely defied Trump, and then typically on matters of foreign policy. A vote to disapprove of his emergency declaration would strike at a cornerstone of Trump’s presidency — his promise to build a wall on the U.S.-Mexico border, which remains overwhelmingly popular with GOP voters.

While Congress considers blocking the declaration, the federal bureaucracy is already lurching into action, searching for money for wall construction. Last week, Congress passed a sweeping spending bill that includes $1.375 billion for border barriers — far short of Trump’s request.

So the administration is looking to take an additional $6.1 billion from the Pentagon budget to supplement that funding.

On Friday, a senior defense official said the Pentagon is exploring ways to shift more than $2 billion into an account that Congress mandated for counterdrug activities. While that account currently contains about $85 million, administration officials said plans were underway to transfer in billions more from other defense accounts.

The official, who was authorized to speak to reporters by the Pentagon on the condition of anonymity, said that while the department has in the past sought approval from Congress for such “reprogrammings,” it is not required by law.

The defense official said the administration plans to designate high-priority sections of the border as “drug smuggling corridors” and will coordinate wall construction in those areas through the U.S. Army Corps of Engineers.

However, the official said it will be months before that happens, and “longer than months for the completion of construction.”

The Pentagon is assessing which projects to freeze or scrap to free up the $6.1 billion Trump has targeted in this year’s defense budget. In the crosshairs: projects that haven’t been awarded to contractors yet and projects aimed at fixing or replacing existing facilities rather than building new ones.

The Pentagon has ruled out taking money from military housing, the official said.

One Democratic congressional aide familiar with the Pentagon plan said that it amounted to “a form of money laundering” but that lawmakers probably have little immediate recourse. Congress has routinely given agencies the power to reprogram unused funds — though those amounts in the past have tended to be in the millions, not billions.

The aide, who was not authorized to comment publicly on the matter, said shifting the funds would create political fallout.

“They are not just going to be sitting around somewhere. You’re going to have to take it away from things that were appropriated for a reason,” the aide said.

The president’s political action committee on Friday began rerunning ads on Facebook with a message targeting Republican senators.

“I want to be able to show all Republican Senators a list of the many American voters that will NOT be happy if the wall isn’t built,” the ads said. “I need YOUR NAME on the list. Sign our Official Petition to the Senate now!”

White House and congressional GOP aides said Friday that there was no concentrated effort on Capitol Hill to lobby Republicans to vote against the resolution of disapproval — reflecting a lack of serious concern that many Republicans would defy the president and join an effort to override his veto. In the House, at least 53 Republicans would be needed for an override, while 20 of 53 GOP votes would be necessary in the Senate.

Josh Holmes, a former top aide to Senate Majority Leader Mitch McConnell (R-Ky.), said that he expects the Senate to pass the disapproval resolution but that he sees little chance of 20 Republican senators breaking ranks. That, he said, could give “flexibility” to a few GOP senators to vote their conscience while the rest of the party holds the line and protects the president’s agenda.

Any political fallout, Holmes predicted, would be fleeting.

“We had a record-setting government shutdown,” he said. “We’ve had a near-constant debate about the merits and demerits of a wall. I don’t think there is a single American voter who has not formulated an opinion one way or another on that issue. So what a vote would do that two years of daily discussion haven’t eludes me.”

Rep. Joaquin Castro (D-Tex.), who authored the one-page House resolution, said that he would continue calling Republicans over the weekend to build support for the measure and that he was hopeful many would vote with Democrats to reject the emergency.

“This is a historic power grab, and it will require historic unity by members of Congress, Republican and Democrat, liberal and conservative,” he said.

With the measure ultimately likely to fail, Democrats are looking at other options to undermine Trump’s emergency declaration.

The House Judiciary Committee has set a Thursday hearing on the matter, and Chairman Jerrold Nadler (D-N.Y.) has invited White House Counsel Pat Cipollone and Justice Department officials to appear. None have yet confirmed their participation, a committee spokeswoman said Friday.

Trump’s move is also being challenged in the federal courts, where several parties — including a coalition of Democratic-led states — have filed lawsuits to overturn the emergency.

Pelosi said Friday that House committees continue to study whether the legislature itself could pursue legal action but have reached no conclusions.

“I’m not announcing anything today,” she said.

Toluse Olorunnipa and Philip Rucker contributed to this report.