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Sen. Mitch McConnell arrives prior to the Senate Republicans weekly policy luncheon, in the U.S. Capitolin Washington on March 25.Al Drago/Getty Images

A bipartisan coalition of U.S. senators have voted in favour of ending the state of emergency that allows the Trump administration to unilaterally impose tariffs on Canadian goods, arguing that the White House overreached by creating an imaginary fentanyl crisis to target its northern neighbour.

The Democrat-led resolution passed 51 to 48 on Wednesday evening, but it is largely symbolic because the Republican-controlled House of Representatives has pledged not to intervene in President Donald Trump’s tariff agenda. He also has veto power that can only be overridden by a two-thirds majority vote in each chamber.

Still, the Senate vote marks the most significant Republican rebuke of Mr. Trump’s second term, with four members of his own party – Senators Mitch McConnell and Rand Paul of Kentucky, Susan Collins of Maine, and Lisa Murkowski of Alaska – breaking rank.

The Democrats, meanwhile, took the vote as a victory that sends a clear message to the White House.

“I challenged Donald Trump’s idiotic Canadian tariffs – a huge tax increase on American consumers that will hurt our businesses – and I won,” Democratic Senator Tim Kaine, who introduced the resolution, said in a video statement late Wednesday.

“On to the House, to see if we can stop Trump’s idiotic trade war from imposing more costs on American families.”

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The resolution was a test of the willingness of members of the Republican Party to stand against Mr. Trump’s invocation of emergency powers to impose tariffs, which are a central pillar of the President’s America First trade policy.

Just hours before the vote, Mr. Trump signed a sweeping executive order stating that existing tariffs he imposed on Canada and Mexico because of their inability to stop the flow of fentanyl would remain in place.

Early Wednesday morning, Mr. Trump warned Republican senators against voting with the Democrats. In a social-media post, he singled out the four Republican senators who had indicated they would support the resolution or were considering it.

The fate of the resolution was unclear until the votes were officially cast. Mr. McConnell, who had earlier in the day declined to comment to The Globe and Mail about his voting intentions, released a statement late Wednesday evening explaining his rationale for supporting the initiative.

“As I have always warned, tariffs are bad policy, and trade wars with our partners hurt working people most,” Mr. McConnell said. “Preserving the long-term prosperity of American industry and workers requires working with our allies, not against them.”

He said broad-based tariffs could have long-term consequences for Kentuckians, who sell crops around the globe and who craft 95 per cent of the world’s bourbon.

The White House has repeatedly cited U.S. Customs and Border Protection data to assert that 43 pounds of fentanyl was seized at the northern border last fiscal year, accusing Canada of being responsible for a “massive 2050% increase” compared with the year prior.

Mr. Trump said in his Feb. 1 executive order that the flow of illicit drugs from Canada constituted an “unusual and extraordinary threat” requiring expanded executive authority to impose tariffs. The emergency declaration unlocked his ability to introduce import taxes without congressional approval.

New data released to The Globe under freedom-of-information laws show that the fentanyl that authorities seized in the northern region and have traced to Canada is significantly less. In its last fiscal year, the border agency determined that 0.74 pounds of fentanyl originated in Canada, according to the data, published Wednesday.

Mr. Kaine said in his remarks on the Senate floor ahead of the vote that the Trump administration invented the emergency to serve its trade agenda.

“Is fentanyl a problem? Yes. Is it an emergency? Yes,” he said. “But fentanyl is not a Canadian emergency.” He pointed to The Globe’s investigation as evidence that the northern-border figure cited by the White House is inflated.

Over the course of the day, senators took to the floor to argue their case on either side. Many spoke to the economic toll of the tariffs on their constituents, and the deterioration of a historically cherished relationship with a country long seen as a friend and ally.

Mr. Paul, who is a staunch supporter of free markets and who co-sponsored the bill, said tariffs in general are a “terrible mistake” and that the declaration at the northern border “is not a real emergency.”

Even before Mr. Trump took office, Mr. Paul had been outspoken about the need to rein in emergency powers. He reiterated those concerns to his colleagues, saying a sitting president’s political party has no bearing on his view that executive powers should not be abused in the name of a self-declared emergency.

Ms. Collins was also among those who broke rank with her party and addressed the Senate ahead of the vote, saying that although she is committed to stemming the tide of fentanyl flowing into the United States, “the fact is the vast majority of fentanyl in America comes from the southern border.”

She said that “unlike Mexico and China, Canada is not complicit in this crisis, and we should continue working with our Canadian allies to secure the northern border, not unfairly penalize them. Our consumers, our manufacturers, our lobstermen, our blueberry growers, our potato farmers will pay the price.”

Those who voted against the resolution doubled-down on the White House’s assertion that fentanyl is “pouring” across the northern border into the United States – despite the fact that the U.S. intelligence community’s annual threat assessment released last week doesn’t mention Canada.

White House spokesperson Kush Desai has defended the emergency declaration.

“Fentanyl seizures along our northern border – which are reflective of the much larger quantities of drugs being smuggled without detection – regardless of the country of origin only reinforce the need for stronger border security and drug enforcement,” he said in an e-mailed statement earlier this week.