The provincial district of Gatineau in Qubec, just across the river from Ottawa, has for decades been a stronghold of Justin Trudeau’s sister party.

But on Monday a little-known upstart populist movement seized the riding from the liberals in a shock election win that has thrown a grenade into the Canadian political system.

The Coalition Avenir Quebec (CAQ) swept up the seat as they were elected as the ruling party of Quebec, jolting the liberal established order that has existed in the Canadian province for nearly half a century, and sparking soul searching among their dejected rivals.

The election marked effectively the first time a populist party has come to power in North America, with immigration…