Several Republican senators are preparing to deliver a major blow to President Donald Trump’s trade agenda by backing a resolution to block his controversial tariffs on Canada. These GOP senators are set to join Democrats in a critical vote on Tuesday, aiming to prevent Trump from imposing sweeping tariffs on America’s closest neighbor and largest trading partner.

Senator Susan Collins (R-Maine) announced Monday that she will support the resolution led by Senator Tim Kaine (D-Va.), which seeks to end the national emergency Trump declared last month to justify the tariffs. This emergency declaration, centered on claims of fentanyl trafficking and illegal immigration, would pave the way for 25% tariffs on Canada—something Trump has vowed to implement as early as this week.

“Imposing tariffs on Canada, which is our closest neighbor, [a] friendly ally, is a huge mistake and will cause disruption in the economies of both countries,” Collins said, visibly concerned as she wore a Canadian flag pin alongside a U.S. flag pin, according to Politico. She warned of serious economic fallout, including job losses in Maine’s key industries like lobstering, wood pulp, and agriculture.

Collins is not alone. Senators Rand Paul of Kentucky and Thom Tillis of North Carolina, both staunch critics of tariffs, are also voicing strong opposition. Paul, who co-sponsored Kaine’s resolution, has been a fierce opponent of Trump’s tariff policies, while Tillis has raised alarms about the impact on U.S. relations with neighboring countries. Even Senator Chuck Grassley of Iowa, representing a state reliant on agriculture, has expressed concerns about the tariffs, though he has not yet made a final decision.

Collins has made it clear that while her support hinges on a final review of the resolution, she agrees with its intent. For the resolution to pass, at least four Republicans must break ranks with Trump and join the Democrats, who are expected to unanimously support the measure.

While the resolution is unlikely to reach a vote in the House, where Speaker Mike Johnson has moved to block such efforts, the vote in the Senate would still represent a significant challenge to Trump’s trade policies.

If the resolution passes, it would be a stunning rebuke to Trump from Republicans—something that hasn’t happened on this scale in his second term. Up to now, GOP lawmakers have largely fallen in line with Trump’s aggressive agenda, including his drastic cuts to federal spending and contentious foreign policy moves.

Senate Majority Leader John Thune, however, appeared uncertain about the resolution’s prospects, acknowledging concerns over the broader impact of tariffs. “We want to give the president as much latitude as possible,” Thune said. “But when it comes to across-the-board tariffs on Canada, I’m in a very different place.”

For many Republicans, the idea of imposing tariffs on Canada—an ally and long-time trade partner—has pushed them past the breaking point. The threat of Trump’s new tariffs has also shaken up the markets, causing increased nervousness among GOP lawmakers. On Capitol Hill, Trump’s chief legislative aide, James Braid, has been working overtime to calm worried Republicans ahead of the planned tariff rollout.

Tillis expressed the frustration many Republicans feel, saying, “We need to fight battles with our foes first and then try to figure out any inequities with our friends second.” He continued, “You want to go after China, there’s not enough tariffs that could ever anger me. Canada and Mexico, that’s a little different.”