Open this photo in gallery Mr. Trudeau only served a year as prime minister with Mr. Obama in the White House, but they hit it off. Kevin Lamarque/Reuters

It’s something we might have expected from norm-breaker Donald Trump, not the highly ethical Barack Obama.

The closest another president came to making the type of disruptive intervention in a Canadian election that Mr. Obama did for Justin Trudeau on Wednesday was way back in 1911.

Prime Minister Wilfrid Laurier was in a dogfight with Tory Leader Robert Borden. A reciprocity agreement with the United States was the critical issue.

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The Laurier team asked President William Howard Taft to make a speech repudiating fears that such a deal would lead to annexation. A blockbuster address was prepared. But in the final days, Mr. Taft, hearing the Liberals would likely win without it, backed down. He and Mr. Laurier came to regret the decision. The Borden Tories won.

The Obama intrusion, unprecedented even for a former president, constitutes unjustified meddling. “The world needs his progressive leadership now,” Mr. Obama said of Mr. Trudeau, “and I hope our neighbours to the north support him for another term.” That it came from someone such as Mr. Obama, who has a reputation for playing by the norms, makes it all the more surprising.

The move, which justifiably infuriates Conservatives, is a gift for the Liberals who could receive a crucial shot of momentum from it in these, the closing days of a very tight race.

What gives the endorsement added importance is Mr. Obama’s high standing in Canada. Polls have always shown him to be admired among a broad swath of the population, not just progressives. “Certainly should give the campaign a boost,” said David MacNaughton, who served the past four years as ambassador to Washington. “Obama’s still very popular in Canada.”

That the former president would go to the extent he did reflects the new era of hardened partisanship and polarization we live in. It also is confirmation that the Liberals were in serious trouble in the campaign.

The worst example of American interference in a Canadian election was what the Kennedys did to the John Diefenbaker Tories in 1963. John F. Kennedy loathed Mr. Diefenbaker. But even at that, he did not go so far as to publicly endorse Liberal Leader Lester Pearson.

Instead, the Kennedys went to unusual lengths behind the scenes to help the Grits. They even sneaked their ace pollster Lou Harris across the border, outfitting him with a false passport, to work for Mr. Pearson. In addition, Kennedy buddy Ben Bradlee, then at Newsweek, later editor of The Washington Post, presided over a negative Newsweek cover story on the Chief. It featured a jacket image of him looking psycho.

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Mr. Trudeau only served a year as prime minister with Mr. Obama in the White House. But they hit it off. Former Obama adviser David Axelrod formed a close relationship with top Trudeau strategist Gerry Butts and provided advice to the Liberals when they toppled Stephen Harper in the 2015 election. Mr. Butts hosted Mr. Axelrod last year when he came to Ottawa to speak to an assembly of progressives.

Until now, there had been silence from the U.S. on the Canadian campaign. For his part, Mr. Trudeau had avoided talk of bilateral relations. It was surprising given that he has been credited by most observers as doing a reasonably good job in dealing with a terribly difficult president.

But after the debates last week when the subject of Mr. Trump was strangely not even raised, that all changed. On Monday, Mr. Trudeau boasted in a speech in Windsor, Ont., of having stood up to Mr. Trump in negotiating the new NAFTA and in fighting against steel and aluminum tariffs. Then the Obama endorsement.

Conservative Leader Andrew Scheer has avoided fellow conservative Mr. Trump because of the President’s exceeding degree of unpopularity in Canada. In response to the Obama endorsement of Mr. Trudeau, the last thing the Conservatives would want is a Trump endorsement of Mr. Scheer.

For Mr. Obama, it’s not the first time he’s endorsed a progressive leader. He did so for Emmanuel Macron when he was running for the presidency of France in 2017.

Presidents or past presidents have sometimes spoken out on issues impinging on Canadian elections. Theodore Roosevelt, president from 1901 to 1909, did so in Toronto in 1917 endorsing Conscription. Bill Clinton took on the separatists in a Mont-Tremblant speech in 1999. But they weren’t direct person-to-person campaign endorsements.

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Conservatives can hope that the Obama move will be seen as unjust interference and stir a voter backlash. Coming from another president, it might. From Mr. Obama, it’s unlikely.

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