OTTAWA—Canadian Prime Minister Stephen Harper ordered the country’s Parliament to be dissolved, kicking off an extended election campaign in which the Conservative leader will vie for a fourth term amid a deteriorating economy.

The move, announced Sunday, doesn’t alter the timing of Canada’s scheduled Oct. 19 elections, but it lengthens the official campaign season to 11 weeks—more than double that of recent Canadian races. The longer period plays to the Conservative Party’s strengths because it has a far bigger war chest...