US President Donald Trump's trade war and annexation threats have upended Canada's election and improved the fortunes of the Liberal Party, which could win a fourth consecutive term in power next week.

The Liberals and the country's new prime minister, Mark Carney, looked headed for a crushing defeat in Monday's election until the American president started attacking Canada's economy and even threatening its sovereignty, including suggesting it should become the 51st state.

Trump's dumping on Canada has infuriated its people and stoked a surge in Canadian nationalism that has helped the Liberals flip the election narrative.

"Trump has immersed himself into our lives and has defined the ballot question," former Quebec Premier Jean Charest said.

"What Trump has done is shed light on who we are," he said.

Even separatists in French-speaking Quebec "are very much aligned with other Canadians in defending the country and responding very firmly to the fact that we're not going to be the 51st state," Charest said.