Gilles Duceppe, who led the Bloc Québécois through six elections before resigning his post after his 2011 defeat, will lead the party into one more election campaign.

A formal announcement will be made today at which Mario Beaulieu, leader of the Bloc for less than a year, is expected to cede his place to Duceppe.

What does Gilles Duceppe's comeback appearance mean for the Bloc's future in Quebec...and the NDP's prospects in the federal election? 4:10 Undoubtedly, we will find out soon enough what effect Duceppe's return will have on federal voting intentions in Quebec.

But is there anything we know today that can give us a hint?

The last instance of a poll with Duceppe's name on it comes from Léger in May 2014, shortly after the Parti Québécois's defeat in the April provincial election. The poll asked about voting intentions depending on who was leading the PQ. Gilles Duceppe was one of those options and he scored better than anyone else, boosting the PQ by 10 points to 29 per cent.

At the time, the Bloc Québécois had not yet chosen Beaulieu as leader and was polling at around 21 per cent in Quebec. It is currently polling at just under 17 per cent.

But what if Duceppe manages to boost the Bloc's score to the 29 per cent he managed as a hypothetical PQ leader in 2014?

It might not be an unreasonable target. Before deciding to make today's jump, a poll was commissioned testing Quebecers' voting intentions with Duceppe as leader. According to sources reported by La Presse, Duceppe did almost three times as well as Beaulieu. Presumably that points to a result well over 30 per cent for Duceppe, considering Beaulieu's current polling levels. That suggests 29 per cent is a plausible score.

If the Bloc is boosted to 29 per cent, where do those new votes come from?

According to EKOS's most recent "second-choice" polling, the Bloc is a much more popular second-choice option to NDP supporters than to either the Liberals or the Conservatives.

But without exact numbers, let us instead take a more agnostic approach, and reduce support for the other parties proportional to their current standing in the polls. As the NDP is leading in Quebec, this has the effect of penalizing them the most, which is in line with the second-choice polling from EKOS.

Based on these assumptions, and using ThreeHundredEight.com's seat projection methodology, the Bloc would tie the NDP at about 29 per cent support in Quebec, with the Liberals at 22 per cent and the Conservatives at 15 per cent. This would likely deliver 17 to 40 seats for the Bloc, with the NDP taking 13 to 39 seats. The Liberals would capture 13 to 18 seats and the Conservatives between eight and nine.

The wide range of outcomes for the NDP and Bloc demonstrates how a large number of seats become swing ridings between the two parties at these levels of support. The most likely outcome would be the two parties taking about 25 to 30 seats each.

Even modest Bloc gains could hurt NDP

But a surge to 29 per cent might be a best-case scenario for the Bloc. Only 23.4 per cent of Quebecers voted for the party when it was last led by Gilles Duceppe, in 2011.

With a more modest boost to match that 2011 score, and using the same methodology as before, the NDP would take 31 per cent of the vote and between 38 and 53 seats. The Liberals take 24 per cent and between 13 and 20 seats, with the Bloc capturing one to 13 seats and the Conservatives between eight and 10 seats, with 17 per cent support.

In this scenario, the NDP still stands to win a majority of seats in Quebec but there is the potential for a much poorer performance than in 2011. For the Bloc, it is a better outcome than where the party was heading under Beaulieu. But it is not exactly earth-shattering.

Nevertheless, this does put a serious damper on the NDP's chances of winning a plurality of seats nationwide.

The recent three-way race is predicated on the NDP having Quebec in its back pocket. Without those seats, the party drops to third place in the overall seat count. And if Duceppe manages to have a real impact, pushing his party to the high-20s or more, then an NDP plurality is out of the question. The likelihood that the NDP and Liberals could combine for a majority of seats is also greatly reduced.

Duceppe was dealt a big blow in 2011, but he does give the Bloc a lot more credibility than it had under Beaulieu. He is an experienced campaigner and a familiar face, neither of which could be said about Beaulieu, whose leadership all but confirmed the Bloc's also-ran status. Duceppe will also have an easier time attracting decent candidates, whereas Beaulieu was struggling to nominate a full slate.

These factors may not all add up to a return of the Bloc to its former dominance of Quebec, but they certainly complicate matters for the New Democrats. Nearly every seat a Duceppe-led Bloc wins over what Beaulieu would have managed is a seat-loss for the NDP. And with polls being as close as they are now, every single seat matters.

ThreeHundredEight.com's vote and seat projection model aggregates all publicly released polls, weighing them by sample size, date, and the polling firm's accuracy record. Upper and lower ranges are based on how polls have performed in other recent elections. The seat projection model makes individual projections for all ridings in the country, based on regional shifts in support since the 2011 election and taking into account other factors such as incumbency. The projections are subject to the margins of error of the opinion polls included in the model, as well as the unpredictable nature of politics at the riding level. The polls included in the model vary in size, date, and method, and have not been individually verified by the CBC. You can read the full methodology here.