MONTREAL—Depending on who one talks to, the NDP’s first Quebec leadership debate drew anywhere from 250 to 400 people on Sunday afternoon in Montreal.

Those are decent enough numbers considering that the party could not fill a 500-seat hall in Quebec City for one of the marquee debates that led to the election of Thomas Mulcair as Jack Layton’s successor in 2012.

Back then the NDP held 59 Quebec seats. It is currently down to 16.

But for the Quebec left, the main event on Sunday took place in a different Montreal venue.

For Sunday also marked the official entry on the provincial stage of Gabriel Nadeau-Dubois, a political activist who is widely seen as the rising star of the Quebec left and the latest existential threat to the Parti Québécois.

On Sunday Nadeau-Dubois was acclaimed as the Quebec Solidaire candidate in the Montreal riding of Gouin. Hundreds of well-wishers were on hand for what amounted to a passing of the torch.

Nadeau-Dubois is inheriting both the riding and the leadership position of party founder and co-leader Françoise David.

If Nadeau-Dubois’ name sounds familiar it is because he was a leading figure of the 2012 Quebec Maple Spring. Two years later he parlayed the experience of leading a massive student strike into an essay that earned him the Governor-General award for French non-fiction. (He donated the prize money to the coalition that is fighting TransCanada’s Energy East pipeline project.)

Of the three student leaders who led the battle against a planned hike in tuition fees five years ago, Nadeau-Dubois is the last to try to parlay his notoriety into a political career on the sovereigntist side of the Quebec divide but the only one to choose to do so outside the conventional PQ/Bloc fold.

That is not for lack of being courted by the sovereigntist establishment.

As recently as the 2015 federal campaign, former Bloc Québécois leader Gilles Duceppe tried to convince him to run under his banner.

Premier Philippe Couillard has yet to call a byelection in Gouin. When he does, Nadeau-Dubois can expect a smooth ride to a seat in the National Assembly.

Uncharacteristically, the PQ has opted to take a pass at running a candidate against him.

That could be described as making a virtue out of necessity.

In theory the PQ decision is a goodwill gesture designed to facilitate the negotiation of a non-aggression pact between the two parties in time for next year’s general election.

In practice, the PQ tested some names in Gouin — a party safe seat until 2012 — and found that none of its prospective candidates had a solid shot at beating Nadeau-Dubois.

PQ Leader Jean-François Lisée’s own riding is next door. Absent a pact between the two parties, he could have a fight on his hands next year.

But finding the common ground for an alliance could be easier said than done.

Lisée has put the sovereignty agenda on hold until a second PQ mandate. In the best-case scenario for the party, it will be six years, at the earliest, before the issue is revisited in a referendum. In the meantime, the goal of a sovereign Quebec is too distant to provide a rationale for an election truce between pro-independence parties.

Absent that common cause, Lisée would have to tilt his party platform sharply to the left and agree to abandon an unspecified number of winnable seats to its junior QS partner to bring it on side. Nadeau-Dubois also has no time for the charter-style identity politics that the PQ has branched into over the past few years.

But Lisée can only shore up his left flank in Montreal at the risk of making his right flank more vulnerable to the conservative Coalition Avenir Quebec in the rest of the province.

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On balance, Nadeau-Dubois’ arrival on the scene is likely to compound the erosion of the PQ base. Over the past 14 years, a succession of party leaders have tried and failed to reverse that erosion.

Is Quebec’s prime sovereigntist party set up for a fall of the kind that nearly wiped the Bloc off the federal map in 2011? No Quebec insider is ready to make that leap. But what is certain is that the Parti Québécois is no longer the de facto natural home of ambitious young sovereigntists.

Chantal Hébert is a national affairs writer. Her column appears Tuesday, Thursday and Saturday.

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