There’s a trick, long known to certain politicians, to get an electoral boost when you’re down in the polls: You declare that dangerous people are about to come across the border, and you latch onto a conspiracy theory claiming that the other political party, or some dark forces associated with them, are responsible.

It can be an effective tactic. Immigration is often a popular election issue, especially when it’s mixed with atavistic fears of mysterious predators entering your territory. It is also a profoundly dangerous tactic.

On Wednesday night, we heard the U.S. President attempt this trick, for the umpteenth time. Americans, Donald Trump declared in an address, are being “raped, murdered and beaten to death with a hammer” by nefarious figures streaming across the southern border, and “thousands more lives will be lost if we don’t act right now” to build his wall.

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Never mind that the threat is an utter fiction – illegal border crossings from Mexico to the United States are at their lowest rate in almost half a century, and those who make the crossings are measurably less murder-prone than Americans.

It’s also based on a wild conspiracy theory. Mr. Trump has repeatedly told voters that migrants approaching the U.S. border include “criminals and unknown Middle Easterners,” as well as terrorists, even though his own immigration officials deny this. He’s said that their march on the border is being funded by mysterious Democratic-linked forces; in October, he publicly endorsed an anti-Jewish conspiracy theory blaming Hungarian-American philanthropist George Soros for the “caravan.”

But Canadians can’t watch this with any sense of superiority. For the first time in decades, this tactic has crept into mainstream Canadian politics.

Conservative leader Andrew Scheer shocked many members of his own party last month by taking up a cause that had emerged from the fringes, denouncing a United Nations document known as the Global Compact for Safe, Orderly and Regular Migration.

That document, if you bother to read it, is an anodyne, purely symbolic statement of principles intended to reduce overall immigration numbers, and especially to discourage irregular – that is, illegal – immigration. Like other such UN compacts, its main purpose is to provide principled-sounding statements for preambles of other documents.

Instead, Mr. Scheer claimed that the Compact “gives influence over Canada’s immigration system to foreign entities." He then denounced the “crisis at our borders” and “chaos at our borders” caused by “illegal border crossers” – suggesting that cross-border chaos, danger and criminality would be products of this document.

Where did this weird theory come from? As Laurens Cerulus and Eline Schaart found out in an investigation this week for Politico, it was the product of a calculated social-media campaign by “a coalition of anti-Islam, far-right and neo-Nazi sympathizers” based in Europe. It was taken up in September by far-right parties in Europe, and by figures in Mr. Trump’s circle.

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Mr. Scheer’s decision to join Mr. Trump in picking up this ugly thread might have seemed like an expedient way to turn immigration fears into anti-Liberal sentiment. Yet, the larger danger of such conspiracy theories is not just that they are absurdly false – but that some people really believe them.

In October, 11 people were shot to death in a Pittsburgh synagogue by a man shouting anti-Semitic slogans. To judge by his social-media posts and statements, the alleged shooter, Robert Bowers, had come to believe that criminal migrants headed to the Mexico-U.S. border were being funded and supported by Mr. Soros and other Jewish figures and organizations – the same conspiracy theory Mr. Trump endorsed. A few days earlier, a Trump supporter in Florida had sent pipe bombs to Mr. Soros and other Democratic-linked figures in apparent support of this theory.

These incidents, and others like them, followed a 2011 massacre in Norway orchestrated by Anders Breivik, who killed 77 people – many of them children – because he had come to believe a theory, promoted by European right-wing politicians, that “globalists” and “cultural Marxists” (including his victims) were conspiring to bring in threatening Muslim immigrants.

That conspiracy theory has now reached Canada. In January, 2017, Alexandre Bissonnette walked into a Quebec City mosque and shot 19 people, killing six. In his police interview, he said he had been spurred to action after watching reports about Mr. Trump’s proposed Muslim ban, and after hearing conspiracy theories about Canada’s Syrian refugees. “I saw that and I like lost my mind,” he said. “I don’t want them to kill my parents, my family."

Nobody but these killers themselves are responsible for their actions. But they all had been led to believe fictions about border-crossing bogeymen and the figures who supposedly back them. Given the dangerous implications of such inventions, to amplify them in the name of momentary political gain wouldn’t just be profoundly unwise. It would be absolutely reckless.