The Canadian prime minister met with Governor General Julie Payette, the acting head of state, who formally gave her blessing for the launch of a six-week federal election campaign.
“Canadians will head to the polls on 21 October,” Mr Trudeau said outside Ms Payette’s official residence in Ottawa.
“We’ve done a lot this past four years, but the truth is we’re just getting started.”
He said Canadians need to decide whether they want to go back to the failed policies of a conservative government that believes in cuts and austerity.
Mr Trudeau is admired abroad for his progressive politics but has been damaged by scandals at home.
Opinion polls suggest the opposition Conservative party is running close behind Mr Trudeau’s Liberals.
Not since 1935 has a government that won a majority of seats in parliament in its first term lost power in the next election in Canada.
Mr Trudeau, who swept to office in November 2015 promising “sunny ways” and stressing the importance of gender equality, gay rights and the environment, faces an electorate more focused on the economy and affordability
But he may not win enough seats to govern by himself after a series of missteps that called into question his leadership while cutting into his once sky-high popularity.
It would leave him and his Liberal Party weakened, relying on opposition members of parliament to push through legislation.
A Nanos Research poll released on Tuesday showed the Liberals at 34.6 per cent and the Conservatives, led by Andrew Scheer, at 30.7 per cent. That margin would not be enough to guarantee a majority of seats in the House of Commons.
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