Republican Senator Thom Tillis on Monday evening announced his support for a Democratic resolution to block Donald Trump’s emergency declaration, bringing the president to the threshold of a humiliating defeat. In a Washington Post op-ed, Tillis said he supports Trump’s border-security agenda, but is concerned about the precedent such an end around would establish. “As a U.S. senator, I cannot justify providing the executive with more ways to bypass Congress,” Tillis wrote. “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.”

Trump cannot afford one more defection in the Senate, or he’ll be forced to issue his first veto. Republicans Susan Collins and Lisa Murkowski have already announced their support for the resolution, which will almost certainly pass the Democratically controlled House on Tuesday. The G.O.P. controls the Senate, but four Republicans voting with the Democrats would be enough to send Joaquin Castro’s resolution to the president’s desk—a rebuke that would prove embarrassing for Trump.

Presumably eyeing the narrowing gap, the president has begun lobbying his party ahead of the vote, tweeting Monday that he “hope[s] our great Republican Senators don’t get led down the path of weak and ineffective Border Security.” “Be strong and smart,” Trump wrote. “Don’t fall into the Democrats ‘trap’ of Open Borders and Crime!” Democrats, too, have been working to drum up support behind the scenes, Politico reports, noting that leaders have circulated a spreadsheet to Republicans outlining the various military projects the president could pull money from to fund his wall. That may be helping to goose Republicans’ concerns about Trump’s so-called “emergency,” but it’s not clear whether it will be enough to turn them against the president’s overreach.

As Politico noted Monday night, even Republicans who have expressed opposition to Trump’s use of the nuclear option—John Cornyn and Chuck Grassley, for example—have signaled they’ll stand with the president. Other Republican critics of the move, like the wishy-washy Marco Rubio, claim not to have read the one-page resolution. Others told the outlet that they “despise” Trump’s unilateral move as much as Tillis does, but are wary of running afoul of the president. “I didn’t want it to come to this, because I think it’s probably going to get tied up in court. And I said it wasn’t a practical solution,” Cornyn said. “I’m probably going to come down on the side of voting against the resolution of disapproval.”

That means the most viable path to blocking Trump’s emergency declaration is likely the courts, where several states have already filed a lawsuit against the administration. Still, a formal rebuke of the president by Congress would be a significant symbolic victory, and a further sign of bipartisan opposition to what Nancy Pelosi has slammed as a “fake emergency.” “Every Member of Congress swears an oath to protect & defend the Constitution,” Pelosi tweeted early Monday evening. “We must not abandon that responsibility now.”

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