Sen. Thom Tillis (R-NC) announced he’ll vote to undo President Trump’s border wall emergency declaration Monday evening, making him the third Senate Republican to do so and leaving the Senate just one vote short of passing a major rebuke to the President.

“As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress. As a conservative, I cannot endorse a precedent that I know future left-wing presidents will exploit to advance radical policies that will erode economic and individual freedoms,” Tillis wrote in an op-ed in the Washington Post.

He joins Sens. Lisa Murkowski (R-AK) and Susan Collins (R-ME) in planning to vote to block Trump’s decision to declare a national emergency along the U.S.-Mexico border and seize money that Congress appropriated for other defense construction, putting the measure on the brink of passage. More than a dozen other Senate Republicans have voiced some degree of displeasure with Trump’s move, and if one more defects from their party, an impending Senate resolution will pass the Senate.

That wouldn’t have a ton of real-world impact — Trump’s move has already been tied up in the courts, and Trump will veto the resolution if it passes. But it would be an embarrassing setback for the President, generating new headlines about his unpopular move. It would also mark the first time a significant chunk of Republicans decided to buck Trump on a major issue.

Tillis faces a potentially competitive reelection in his slightly Republican-leaning state, and may have one eye on 2020. Others who face the same concern may make similar moves, as may more institutionalist-minded Republicans who have little to fear from Trump. The House is expected to pass the measure on Tuesday, and the Senate vote will most likely happen sometime before mid-March.