Conservative Leader Pierre Poilievre has written a letter to his NDP counterpart asking Jagmeet Singh to pull his party's support for the Liberal government so Canadians can go to the polls this fall instead of next year as planned.

"Canadians can't afford or even endure another year of this costly coalition. No one voted for you to keep Trudeau in power. You do not have a mandate to drag out his government another year," Poilievre wrote in his letter.

"Pull out of the costly coalition and vote non-confidence in the government this September to trigger a carbon tax election in October of THIS YEAR. Or you will forever be known as 'Sellout Singh,'" Poilievre said.

Poilievre's challenge to Singh comes as the parties square off in a federal byelection in Manitoba, a Sept. 16 vote that is expected to be a competitive two-way race between the Conservative and NDP candidates.