The Liberal convention in Ottawa is a replay of the party’s greatest hits

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Sometimes, an easy path to friendship is realizing that you both hate the same person, and then standing side by side hating that person as hard as you possibly can, together. It’s a sugar high of empty calories, and it will rot your teeth and your bond eventually if you’re not careful, but it often does the job quite deliciously for a while.

This is what Pierre Poilievre and his Conservatives have been doing for some time now. They have so ferociously and gleefully and unanimously hated Justin Trudeau that it never seemed to occur to any of them that eventually you have to give people a reason to like and trust you, not just suggest someone else to launch poison-tipped darts at.

On Thursday, Mr. Trudeau and the Liberal faithful gathered for their first in-person party convention in five years, the whole thing revved up with the nostalgia of the first day back at summer camp. And then, with Mr. Trudeau’s opening-night keynote speech – he’d been reduced from headliner at the crescendo of the convention to the warm-up act because he was headed to London for King Charles’s coronation – the Liberals too indulged in the savage joy of loathing the same guy together.

In Liberal eyes, as Mr. Trudeau told it, Mr. Poilievre is an unsuitable leader – really not a leader at all – because of his support for the convoy protest, his dalliances with odious views and entities, a suspicion that he doesn’t support abortion rights, the fact that he’s more interested in whipping up anger than building up Canadians and Canada itself.

On and on it went, a festival of shared contempt between a few thousand party faithful in a cavernous convention hall and their leader up on the stage, all of them elbowing each other in the ribs to get a load of this Pierre guy.

“They either say investing in Canadians is a waste of money, or that our policies are too woke. Too woke!?” Mr. Trudeau drawled out incredulously. Then he squared himself to the camera that was projecting his image on the massive screens at the front of the hall and snarled, “Hey, Pierre Poilievre, it’s time for you to wake up.”

The crowd leapt to its feet, hooting and applauding.

The point of a political hate-fest, of course, is not just that you and your partisan friends find a certain person awful, but that you hope large numbers of other people do too, or that they can be encouraged to do so if you keep pointing out the relevant faults. In that regard, there was a dual audience for Mr. Trudeau’s speech on Thursday night: the Liberal faithful in the room, who need to be kept in a state of rapturous readiness between campaigns, and the Canadian public who would presumably see the juiciest clips from the speech.

“More than ever, in this consequential moment in the world, your energy is needed – all of you in this room and all of you across this country,” Mr. Trudeau said.

Like all incumbents, he and his party have to tread a tricky line, on both the small and friendly scale of this convention and with the rest of Canada whenever the next federal election comes. The Liberals must make the case that they have steered the ship well during their nearly eight years in power, without seeming oblivious to the economic vice many Canadians feel squeezing their chests on a daily basis. And the party has to paint this moment as too fraught to entrust to anyone else, without admitting that maybe things aren’t going so well, which would land in their laps as the people in charge.

Maybe as a way to sidestep some of that messiness, the set dressing at this convention seemed designed to recall the recent good old days, while perhaps suggesting we can see them again.

Before Mr. Trudeau took the stage for his speech – 30 minutes late, because that’s how he rolls – a sort of movie trailer of campaign highlights played to pump up the crowd. Then came the real thing, a reminder that – among this loyal and friendly crowd at least – Mr. Trudeau’s strongest gear as a politician remains his talent for conducting and riding the energy wave of a big crowd.

Twice during his speech, someone in the hall spontaneously yelled out, “We love you!”

One floor down, on the main concourse of the downtown Ottawa convention centre, another huge screen played a different sizzle reel of campaign moments: Mr. Trudeau in a black leather jacket hollering ecstatically at a crowd; leaning out from a stage toward a seaweed bed of beckoning arms; working his way down arena steps tossing out high fives.

Everything in the video suggested everlasting momentum, public enthusiasm that never dimmed or tarnished, all of it paused mid-air in the golden glow of 2015. Remember this? We were really good at this. He was really good at this. We could do it all again.

The video played silently on a perpetual loop, a campaign that never ended.