MONTREAL — When the campaign for Canada’s federal elections began in August, all indicators seemed aligned for Tom Mulcair and his New Democratic Party. Historically a distant third in Parliament, the New Democrats led in many polls. The party’s profile had risen in May after its Alberta arm ended four decades of Conservative rule in that province, the stronghold of Prime Minister Stephen Harper.

But as Monday’s vote approaches, the prospect that Mr. Mulcair, 60, will lead the New Democrats, a left-of-center party founded partly by organized labor 54 years ago, to power for the first time has faded.

If the effort fails, there will be no single cause to blame. Justin Trudeau, in his election debut as Liberal leader, has run an unexpectedly strong campaign, and Mr. Harper has made the veils worn by Muslim women a potent issue. And many say Mr. Mulcair seems uncomfortable at times on the trail.

Then there is the argument that the party’s initial optimism rested on false hopes.

Richard Johnston, a political scientist at the University of British Columbia in Vancouver, attributed the New Democrats’ rise in the polls not to Mr. Mulcair but to the provincial leader Rachel Notley’s victory in Alberta in May.