Canadian voters head to the polls Monday to elect their next prime minister, and politicos on both sides of the border see the long shadow of President Trump on the ballot — either as an unexpected boon for the ruling Liberal Party or a potential spoiler for his ideological counterparts, the Conservative Party. 

By any measure, Mr. Trump has played an outsized role in this year’s Canadian elections. Even before his inauguration, he threatened 25 percent tariffs against America’s largest trading partner. Shortly after taking office — and again, repeatedly, in the months since — he taunted Prime Minister Trudeau about making Canada the 51st American state. 

The taunts backfired, however, after the highly unpopular Mr. Trudeau, who was trailing by double digits in polling against Conservative Party leader Pierre Poilievre, resigned, forcing his Liberal Party to hold leadership elections in March.

The winner of the leadership contest, former Canadian central banker Mark Carney — who also served as the governor of the Bank of England from 2013 to 2020 — managed to eliminate the once formidable lead by ginning up national pride over Mr. Trump’s comments. 

“This is Canada, we decide what happens here,” Mr. Carney said at a campaign event with manufacturing laborers last week. “President Trump has been attacking Canada, America’s closest ally, America’s closest partner. What President Trump wants to do is to divide because that makes his job easier. He is trying to break us so America can own us. … That will never happen.”

Having blunted some of the agony imposed on Canadians by rescinding Mr. Trudeau’s consumer carbon tax and emissions cap on oil and gas production, the Liberal Party is now pledging $20 billion in new revenue from tariffs on American goods should the Trump administration keep its base tariffs of 10 percent. 

However, Mr. Carney’s claims that Mr. Trump is mistreating Canada are more tactical than true, claims investment banker and chairman of international consulting firm SMI Group LLC, Kenin M. Spivak. Calling Canada an “adept abuser of free trade,” Mr. Spivak wrote in the economic newsletter Zero Hedge that America’s goods and services trade deficit with Canada stood at $53 billion last year, and its goods deficit with Canada was $140 billion in January.  

“Canada will shroud itself in this miasma and continue to play the victim. It’s an act. Canada takes care of Canada,” Mr. Spivak wrote.

Though Mr. Poilievre lost his edge after Mr. Carney’s victory, he has recently regained footing by focusing on promises to lower taxes and increase the new housing construction desperately sought by younger voters concerned about the cost of home ownership. The demographic has rewarded Mr. Poiliviere with a shift toward the right, similar to Mr. Trump’s re-election experience. The movement has helped narrow the polling gap in the closing days of the election to within a few points.

Using the language of Ronald Reagan, Mr. Poilievre, 45, has been campaigning on the question of whether Canadians, especially those in the more conservative Western strongholds of the nation, are better off today than when the Liberal Party took power in 2015. 

“The question you have to ask is after a decade of liberal promises, can you afford food? Is your housing more affordable than it used to be? What is your cost of living like compared to what it was a decade ago, and are you prepared to elect the same liberal MPs, the same liberal ministers, the same liberal staffers all over again for a fourth term?”  Mr. Poilievre asks in an election advertisement. 

Unlike America’s two-party system, an outright majority is unusual in Canada’s multi-party parliamentary structure, but it could happen this election. It takes 172 seats to win a majority in the House of Commons. Polling projections done by Canadian firm Abacus Data show the Liberal Party with a 69 percent chance of achieving a clean win. 

Yet polling is also unpredictable. While polls by Focaldata and Politico show a glide path to victory for Mr. Carney, Conservative Party support may be undercounted, says Focaldata’s chief research officer, James Kanagasooriam, whose data show a tightening of the race when voters are asked who they think their neighbors will choose in Monday’s contest.

“Wisdom of the crowds question hints (25% probability) of a massive underestimate of (Conservative Party Canada) vote,” Mr. Kanagasooriam wrote on X.

If Mr. Carney does prevail on Monday, the Council on Foreign Relations’ Josh Kurlantzick says it will only be the latest indicator of a “Trump Effect” that is negatively impacting Mr. Trump’s allies across the globe. 

“Since that inauguration, the president also seems to be creating a ‘Trump effect’ on many, though not all, foreign elections, especially among countries that have been important U.S. allies and partners. In general, the effect has been to boost the popularity of left or center-left parties in national elections,” he wrote.

In the lead-up to Monday’s vote, Mr. Trump’s secretary of state, Marco Rubio, did not back down from Mr. Trump’s expressed desire to make Canada the 51st state, and suggested that America’s northern neighbor may not outlive a new tariff regime. 

“The president has stated repeatedly he thinks Canada would be better off as a state, and he has said that based on what he was told by the previous prime minister, who said Canada can’t survive unless it treats the U.S. unfairly in trade,” Mr. Rubio said on NBC’s Meet the Press on Sunday. 

“They’ll have their elections this week.  They’re going to have a new leader and we’ll deal with the new leadership of Canada,” he said.