Members of the Republican establishment have not been coy about their disapproval of President Donald Trump’s threat to impose tariffs on Mexico. Now, they’re upping the ante.

Congressional Republicans are discussing holding a vote to block the tariffs, which would be a significant smack in the face to a President who has enjoyed (nearly) monolithic loyalty from his party. According to the Washington Post, as a byproduct, the vote could also end up blocking funds Trump earmarked for his border wall.

Trump’s tariffs hinge on his national emergency declaration, which Congress could overturn with a vote of disapproval. Trump vetoed such a vote in March, but this latest dustup seems to be engendering more widespread anger than the last, dangling the possibility of a veto-proof vote.

Some Republicans who are more wary about ruffling the President’s feathers have opted for a backdoor diplomacy route to get him to change his mind on the tariffs. But now, Trump’s out on a branch — his announcement was an aggressive move to extract some kind of concessions from the Mexican government on immigration (no one seems exactly sure of the specifics of what he wants), and now he’d have to find a way out of his strong-arming while still saving some face.