A year of rapidly slumping poll results for the Liberal Party that has governed Canada for nearly a decade was bad enough. But then there came the almost unthinkable: a defeat in a special election in downtown Toronto, the party’s longtime electoral fortress.

The defeat last year, many analysts believe, triggered the chain of events that led to Justin Trudeau’s resignation as prime minister and the federal election that will be held on Monday.

Voters in Toronto had been vital to keeping Mr. Trudeau and the Liberals in power through three elections. So the loss of a Toronto district — held by a prominent Liberal for 28 years — to the Conservatives was a stunning blow and an omen of worse to come in a general election.

Now, with all 343 seats in the House of Commons to be filled in the general election, the 56 at stake in the loosely-defined Greater Toronto Area will likely determine who will steer Canada through a trade battle with the United States that could deal a devastating economic blow.

The Conservatives had been making inroads in the crucial Toronto area, with about 7 million people, before President Trump upended the electoral landscape by imposing tariffs on Canada.

The two leading contenders to become the country’s next leader are Prime Minister Mark Carney of the Liberals and Pierre Poilievre of the Conservatives.