OTTAWA - The leading candidates vying to replace Canadian Prime Minister Justin Trudeau spent considerable time during a televised debate on Monday discussing the need to stand up to US President Donald Trump.

The next election must be held by October 20 this year and until recently the official opposition Conservatives looked set for an easy win after more than nine years of Liberal rule.

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But the race now is much closer, thanks to Trump's threats to impose tariffs on all imports from Canada.

"Trump represents the biggest threat to Canada since World War Two," former finance minister Chrystia Freeland said during a two-hour French-language debate between the four candidates.

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Freeland, who helped negotiate the US-Canada-Mexico trade deal during the first Trump administration, said the president at that point had vowed to wage economic war against Canada.

"But this time Trump's threats are worse - he wants to make Canada the 51st state," she added. Canada sends 75 percent of its goods and services exports to the United States and would be plunged into a recession if the tariffs persisted.

Ex-central banker Mark Carney, who polls indicate is the front-runner, said Trump would never succeed in making good his threat to make Canada a part of the United States.

"At this moment Canada is facing the worst crisis of our lives ... I know how to handle crises," he said. Carney headed the Bank of Canada during the 2008-2009 financial meltdown and then the Bank of England during the Brexit vote.

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Carney and Freeland both say Canada needs to respond robustly to any Trump tariffs.

Freeland's resignation last December after nine years in cabinet triggered a crisis that eventually forced Trudeau to announce last month that he would step down. The new leader will be named on March 9.

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The Conservatives, who say there is no difference between Trudeau and the two main leadership contenders, issued a news release earlier in the day entitled "Sneaky Carney Must Come Clean With Canadians".

The two remaining candidates are former cabinet minister Karina Gould and retired Liberal legislator Frank Baylis.

The four will hold a debate in English, the other of Canada's official languages, on Tuesday.