Canadian Prime Minister Justin Trudeau's ruling Liberals, trailing badly in the polls, face a struggle on Monday to retain a once-safe seat in a special election where failure to win could boost calls for a new party leader.

The election in the Montreal parliamentary constituency of LaSalle—Emard—Verdun was called to replace a Liberal legislator who quit.

Normally Trudeau's party could count on an easy win there but surveys suggest the race is tight. If the Liberals lose, the focus will fall squarely on Trudeau, who has become increasingly unpopular after almost nine years in office.

Unusually, some Liberal legislators are breaking ranks to call for change at the top. Alexandra Mendes, a Liberal lawmaker who represents a Quebec constituency, said many of her constituents wanted Trudeau to go.

"I didn't hear it from two, three people - I heard it from dozens and dozens of people," she told public broadcaster Radio-Canada last week. "He's no longer the right leader."