Open this photo in gallery Maxime Bernier, leader of the People's Party of Canada, gives the thumbs up at the end of his speech at the launch of his campaign, on Aug. 25, 2019 in Sainte-Marie Que. Jacques Boissinot/The Canadian Press

On Monday, Maxime Bernier sent out a string of tweets attacking Greta Thunberg, the student environmental activist. The leader of the new People’s Party seeks to bring to Canada a poison afflicting many democracies: the collapse of convention.

That approach is working for politicians in other countries. We can only hope it fails here.

Mr. Bernier used to be a responsible politician. He held senior cabinet portfolios in Stephen Harper’s government and came within a few votes of winning the Conservative leadership race in 2017, losing to Andrew Scheer.

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After leaving the Conservatives, Mr. Bernier founded his own populist party which, among other things, questions the settled science on climate change.

With most polls showing his party attracting the support of less than 5 per cent of voters, Mr. Bernier has decided to up the ante by indulging in hateful rhetoric.

In a string of tweets, he accused Ms. Thunberg of being “mentally unstable,” along with other slurs that really don’t need to be repeated.

“She has become an influential figure in a movement that is a threat to our prosperity and civilization,” he concluded. “If she wants to play that role, she should be denounced and attacked.” Ms. Thunberg is 16.

Politics is a rough business, and Twitter is a home for extreme views. But no leader of a Canadian political party has gone as far as Mr. Bernier in demonizing his opponents. Why is he doing it? Because it works. Trashing unwritten rules and constitutional conventions is all the rage these days.

On Tuesday, opposition Parliamentarians, including a former Conservative cabinet minister who crossed the floor, struggled to prevent Prime Minister Boris Johnson from taking the United Kingdom out of the European Union on Oct. 31 without an agreement on the terms of departure – a so-called hard Brexit that could devastate the economy.

To frustrate his opponents, Mr. Johnson has asked Queen Elizabeth to prorogue – or suspend – Parliament next week, an action that House of Commons Speaker Jamie Bercow called "a constitutional outrage.” Mr. Johnson might also try to force an election.

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At a time when the United Kingdom faces its greatest crisis since the Second World War, its Prime Minister is riding roughshod over Westminster’s unwritten constitutional conventions. He is not alone.

United States President Donald Trump has violated every unwritten rule since the day he became president. He openly criticizes the decisions of judges, disrespecting the division of powers. He undermines the autonomy of nonpartisan institutions, such as the Federal Reserve (he relentlessly attacks its chair, Jerome Powell, for not lowering interest rates). And he is contemptuous of a free press.

On the latter front, he was typically vicious on Monday, tweeting that major news organizations “are now beyond Fake, they are Corrupt....In the history of our Country, they have never been so bad!" His targets typically include television networks, The New York Times and the Washington Post.

Whether or not a country has a written constitution, democracy operates on a set of unwritten rules that all sides agree to respect. But that mutual respect is under assault from political opportunists.

In Canada, the erosion is less severe than elsewhere (although experts are still debating whether then-prime minister Stephen Harper abused a constitutional convention in 2008 by asking the governor-general to prorogue Parliament to ward off a non-confidence vote).

But the unwritten rule that sets limits on how far political attacks can go has been eroding for years. Mr. Bernier just trashed it, by lobbing the worst kind of personal insults against a Swedish teenager, simply because he disagrees with her stand on climate change.

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Are there any limits left? There might be. Mr. Trump’s approval rating remains mired at around 40 per cent. Most Americans do not approve of his destructive ways. In Britain, Parliament is robustly defending its right to have the final say on matters of national importance. And only 88,000 people follow Maxime Bernier on Twitter.

The People’s Party leader may have settled one issue. The new Leaders’ Debates Commission has thus far refused to permit Mr. Bernier to join in the official leaders’ debates, saying he lacks sufficient support.

The commission promised to revisit the issue closer to the debates. But after what Mr. Bernier said on Monday, no political leader should be willing to stand beside him on any stage.