OTTAWA (Reuters) - If Republican frontrunner Donald Trump wins the U.S. presidential election in November, it will be "a step back" that makes life tough for Canada, Prime Minister Justin Trudeau said on Tuesday.

       Trudeau, whose center-left Liberals came to power in November 2015, had rocky relations with Trump during his first four-year White House term. In 2018, Trump accused Trudeau of being weak and dishonest.

       "It wasn't easy the first time and if there is a second time, it won't be easy either," Trudeau said in French during a discussion hosted by the Montreal chamber of commerce.

       He added: "But we can't imagine a day when it will ever be easy with the Americans. The main responsibility for any prime minister is to represent and defend Canada's interests ... we've been able to do this very well these past few years."

       Canada sends 75% of its goods and services exports to the United States and is particularly vulnerable to any U.S. shift toward protectionism.

       When Trump came to power he vowed to renegotiate the free trade treaty that bound the United States. Canada and Mexico. Ottawa spent almost two years locked in talks to produce a trilateral pact that largely protected Canadian interests.

       About two-thirds of Canadians surveyed this month said U.S. democracy cannot survive another four years of Trump in power, and about half said the United States is on the way to becoming an authoritarian state, a poll released on Monday said.

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       A Trump win would be "a step back" and a victory for "a populism that reflects a lot of anguish and fury ... without necessarily providing solutions", Trudeau said.

       The Liberals have made fighting climate change a priority and Trudeau said Trump showed little interest in the environment during "the lost four years" of his first term.

       "There are clearly issues where I do not agree at all with Mr. Trump," Trudeau said, citing the climate question.

       The next Canadian election has to be held by September 2025 and the Liberals are trailing their right-of-center Conservative rivals. Trudeau repeated criticism that the party had taken its inspiration from the Trump movement.

       (Reporting by David Ljunggren; editing by Jonathan Oatis)