Conservatives were feeling pretty good about themselves in Vancouver last May at their first national convention since they lost power.

The open, inclusive and generally harmonious gathering helped assuage fears they would fall apart after the only elected leader their party had ever known made his exit. It turned out they didn't need Stephen Harper's rigid discipline to avoid nasty infighting, identity crises or a re-embrace of the fringes that cost them in elections past. Their big tent, welcoming to any small-c conservatives prepared to peaceably co-exist, was built to last.

Less than nine months later, that tent is already being blown over by populist winds from south of the border. An ugly flirtation with the sort of angry, fearful populism that overtook U.S. Republicans threatens to define and divide Conservatives here, and to isolate them and their supporters from other Canadians in a way dangerous for all concerned.

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John Ibbitson: Voting for Conservative leader: What would Stephen Harper do?

In the immediate aftermath of a deadly mosque attack, and amid increased reports of anti-Islamic hate incidents, a good chunk of the Official Opposition has latched onto conspiracy theories about the otherwise rather inconsequential parliamentary Motion 103 – put forward by Liberal MP Iqra Khalid back in December – to condemn "Islamophobia." Rather than stories about a cross-partisan condemnation of bigotry, the Conservatives now stand accused of being on the side of the bigots.

If this week's debate on the motion was a trap cynically set by Justin Trudeau's party, the Liberals could not have anticipated how enthusiastically many Tories would fling themselves into it – pandering to hysteria that the motion would curtail non-Muslims' rights. (To be clear: It's a motion, which means it would not change any laws whatsoever, and endorses very little other than the idea that fear of Muslims is a problem and along with other forms of discrimination merits further study.)

That it has played out this way, despite the Conservatives voting in favour of a similar motion condemning Islamophobia just last fall, is the culmination of twin factors: A leadership contest crowded with uninspiring and opportunistic candidates, paired with the rise of nativist carnival-barkers hoping to build a Breitbart-aping business empire.

The campaign to replace Mr. Harper would have the potential to shake the stable ground the Tories thought they were on, in and of itself. It's never easy for an interim leader to maintain cohesion and discipline.

And with no clear front-runner in the double-digits field, few candidates seem interested in positioning to subsequently appeal to the broader electorate – just the small, disproportionately old and white sliver of it that might be inclined to buy a party membership.

Little could have exacerbated those vulnerabilities quite like the concurrent surge of independent efforts outside the party's structure to rally the most suspicious and paranoid of potential Conservative voters.

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To some extent those efforts are organic, with smaller groups of people alarmed by society's changing complexion finding each other through social media. More so, in English Canada at least, they are spearheaded by The Rebel – an online media outlet/propaganda machine/moneymaking scheme that borrows language and tactics from the dodgier corners of Donald Trump's support base and traffics especially heavily in Muslim conspiracy theories.

Owing partly to the marketing prowess of its "commander" Ezra Levant, and partly to the weakness of the Conservative field, The Rebel seems to excite more people than any one leadership contender. So naturally those candidates now defer to it, in some cases by turning up at Rebel events to try to pick up supporters – as Kellie Leitch, Chris Alexander, Brad Trost and Pierre Lemieux did this week at an anti-M-103 rally – and in others by more quietly opposing the motion for obvious fear of facing backlash. Among 14 candidates, only one – Michael Chong – has been willing to openly support it.

The leader they hope to succeed did not exactly set an unassailable example on such matters – not when, after all his previous softening of his party's rougher edges, he spent his final campaign at its helm trying to win votes by promising restrictions on the niqab, and a tip line to report on "barbaric cultural practices."

But after that election, there was a common sentiment among prominent Conservatives – among them Ms. Leitch, the candidate now most overtly playing to The Rebel crowd – that the Muslim-baiting had been a mistake. In a country in which it's very hard to win without immigrant support, the party had sent the wrong signals; it needed to be more inclusive.

Maybe the hope is to go back to being more inclusive again, once they're done playing to a party membership in which immigrant voters are less needed. And maybe it would be easier to do that if the Liberals weren't themselves trying to score some political points – refusing to support the Tories on a compromise motion that similarly condemns discrimination but omits the word "Islamophobia," and in the case of one Liberal MP on Friday very offensively suggesting that Conservative and Parti Québécois policies were "directly" responsible for the recent mosque attack.

But what we've mostly seen from the Liberals on this so far is what political opponents do: They seize on your moments of vulnerability, to test who you really are and expose characteristics that will come back to haunt you.

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The Conservatives thought they were ready for such challenges last spring. Apparently they were wrong.