WASHINGTON ― The Republican-led Senate looks poised to vote down President Donald Trump’s declaration of a national emergency to find funding to build a wall along the U.S.-Mexico border.

Three Senate Republicans have publicly said they would vote for a resolution of disapproval, and another suggested he would. Only four Republicans are needed, because Democrats control 47 seats and the resolution requires a simple majority of 51 votes.

The Democratic-controlled House has scheduled a disapproval vote for Tuesday afternoon, and the measure is likely to pass. Once the Senate receives the bill from the House, any senator can bring it up for a vote after 15 days and Senate Majority Leader Mitch McConnell (R-Ky.) can’t block it.

Trump has said he would veto the resolution if it passed. Congress could override the veto if two-thirds of each chamber voted yes ― which would be the real test of Republican willingness to stand up to the president and defend their prerogatives as lawmakers.

Trump declared a national emergency two weeks ago, after failing to convince Congress to give him $5 billion to build a wall along the southern border even though arrests for illegal border crossings are lower than they have been previously. The declaration, which would allow Trump to use other funds to begin construction, will be subject to a court challenge in addition to congressional disapproval.

Sen. Thom Tillis (R-N.C.) said voting down Trump’s national emergency declaration isn’t a question of supporting the president ― it’s about protecting Congress and its constitutional powers. Tillis wrote in a Washington Post op-ed that it’s his responsibility “to preserve the separation of powers and to curb the kind of executive overreach that Congress has allowed to fester for the better part of the past century.”

Tillis previously said, however, that he would never vote to override a veto.

“I’m going to defer to the president on the best strategy and I would never vote to override a veto on something that the president didn’t think was the best approach,” he told HuffPost in January.

A spokesperson for the senator did not immediately respond to a request for comment.