MONTREAL—With his party’s competitive position in national voting intentions hanging by a Quebec thread, NDP Leader Thomas Mulcair has more to lose than to gain in Thursday’s French-language debate.

Since the campaign began, a commanding lead in Quebec is mostly what has kept the New Democrats in the game. But the status of front-runner comes at a price and Mulcair will spend the first French-language debate of the campaign on the hot seat.

He absolutely needs to keep alive the prospect of a second Quebec orange wave. Absent a strong showing in the province’s polls, the NDP’s carefully constructed bid for power could crumble like a house of cards.

The party’s narrative hangs on the notion that the New Democrats are best placed to beat the Conservatives next month. On the way to this week’s French-language debate, that contention has been sorely tested.

If all had gone according to plan for the NDP, Justin Trudeau would no longer be a realistic contender for prime minister. With the Liberals lagging too far behind to have a shot at winning the election and the help of a cautious platform there would be an NDP wave in the making in Ontario.

So far, neither has happened.

Eight weeks and two English-language debates later — including one on the central issue of the economy — the Liberal leader is still standing and his party has gained more ground in public opinion, in particular in Ontario, than the NDP.

At this juncture, a slide in NDP support in Quebec could lead to a hemorrhage of support in other regions of the country as non-conservative voters coalesce behind the stronger opposition option.

Until this week, the main liability Mulcair was expected to take in the French-language debate was his support for the Energy East pipeline. That stance pits him against Green Party Leader Elizabeth May and Bloc Québécois Leader Gilles Duceppe, and it does not play well with Quebec’s environmental activists.

But now the NDP leader is coming to the debate with a much larger target on his back.

In Quebec, the debate over whether Muslim women should be required to uncover their faces to take the oath of citizenship has given a moribund Bloc campaign a bit of a kiss of life.

Support for a veil ban runs high in Quebec. In the national assembly, a Liberal move to ban face coverings for public servants and for anyone using provincial government services enjoys multi-party support.

The NDP is not alone in opposing a niqab ban. So do the Liberals. But because the Bloc and the New Democrats fish in the same nationalist pool of francophone voters, Duceppe’s attacks have focused on Mulcair.

Anecdotal and recent polling evidence suggest they are taking a toll.

At this juncture, the NDP’s best hope of riding out the niqab controversy would be for the election conversation to move on to other topics. But with not one but two French-language debates on the agenda of the next two weeks and with the help of BQ and Conservative ads, the niqab issue could be on the Quebec radar to stay.

A good debate for Duceppe on Thursday would be good news for Harper and Trudeau.

Every vote that the Bloc takes back from the NDP helps the Liberals and the Conservatives in their own local battles against the New Democrat tide.

The NDP has owned Quebec since the beginning of the campaign but that could still change. Past history shows that the debates tend to focus Quebecers on their election choices — sometimes with unexpected results.

When he arrived on the set of the French-language debate in the last election, Duceppe had cause to believe he was bulletproof. His party was sitting on its usual comfortable lead. As the only Quebec leader on the set, he had the home advantage.

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So widespread were those assumptions that those who covered the 2011 French-language debate did not come away with the impression that it had been a watershed moment.

A few days later, the NDP jumped in the lead in Quebec, and never looked back.

This year, the roles are reversed, with Mulcair the leader to beat and Duceppe the one to watch.