International relations seldom figure into local politics in Canada. And Thursday’s provincial election in Ontario, where many voters are worried about issues closer to home, like access to health care and the soaring cost of housing, should have been no exception.

But President Trump has upended the electoral dynamic.

His threats against Canada — including to impose crushing tariffs on Canadian exports and to annex the country — have been cast as a defining election issue in Ontario, Canada’s most populous province, as voters head to the polls on Thursday.

“This election is about who we are and what we stand for,” Premier Doug Ford, Ontario’s conservative leader, said in a campaign ad. “So let me be clear: Canada will never be the 51st state. Canada is not for sale.”

The province of 16 million, which represents about 40 percent of Canada’s population, is home to some of the country’s key industries, including automotive, manufacturing and technology. Tariffs would deal the province a deeply painful blow, including significant job losses.

Since Mr. Ford called the election a month ago, he has taken a handful of days from the campaign trail to travel to Washington and make the case why tariffs would be ill advised.

Several polls show Mr. Ford holding a double-digit lead over his closest rivals, with voters regarding him as the best candidate to take on Mr. Trump.