The protester leaped on to the stage, lunged at Chrystia Freeland and screamed within inches of her face.

She didn’t flinch.

Her January campaign launch for leadership of the Liberal Party, and by extension Canada, was disrupted, but the encounter made a point no stump speech could have landed as effectively: She’s unflappable.

Ms. Freeland, a career journalist from Alberta who rose through elite institutions to become a top politician, is now running to replace the man who brought her into politics, Justin Trudeau.

On Sunday, Canada’s Liberal Party will announce the results of its election for a new leader, chosen by 400,000 members. The winner will also become Canada’s prime minister, though not for long: The party does not command a majority in Parliament, so has a weak grip on power. Federal elections must take place before October.

Ms. Freeland’s dramatic December resignation as finance minister, deputy prime minister, and all-around right-hand woman to Mr. Trudeau triggered his own decision to step down, plunging Canada into political turmoil.

This has come as Canada is thrust in crisis. This week President Trump made good on his threat to impose sweeping tariffs on Canadian goods and, while he eased some of those measures Thursday, he made plain he would continue to hit Canada’s economy with surcharges.