Denis Lebel, deputy leader of the Conservative Party and former minister in Stephen Harper's government, will be resigning his seat of Lac-Saint-Jean over the summer. When a vote is held to replace him, it could be a tough test of Andrew Scheer's new leadership of the Conservatives.

It also will provide a glimpse of where the Liberals and NDP stand in the province — and whether the Bloc Québécois has any future under its new leader, Martine Ouellet.

Lac-Saint-Jean, a riding north of Quebec City in the Saguenay–Lac-Saint-Jean region, was first won by Lebel in a 2007 byelection when the riding was known as Roberval–Lac-Saint-Jean.

Lebel has since been re-elected three times, most recently in 2015. But his share of the vote in that election fell by nine points. With 33.3 per cent of ballots cast, Lebel just narrowly defeated the NDP's Gisèle Dallaire, who captured 28.5 per cent of the vote.

The Bloc and Liberal candidates trailed with 18.4 per cent of the vote apiece.

But with Quebec now showing some of the most volatile poll numbers in the country, the riding could be a toss-up whenever the byelection is called — and all four parties will have something to prove.

Scheer's first real test?

Lac-Saint-Jean might prove a real challenge for Scheer. It won't be his only electoral test, however, as a byelection will be needed to replace Rona Ambrose in the Alberta riding of Sturgeon River–Parkland.

But that is a Conservative stronghold. In Lac-Saint-Jean, Scheer will be tasked with retaining a close seat in a province that helped deliver him the party's leadership.

Lebel was a former mayor of the town of Roberval within the riding and his candidacy was instrumental in the party's 2007 win there. Since 2011, Lebel has been the sole Conservative MP in the Saguenay–Lac-Saint-Jean region.

His decline in support in 2015 — when the Conservatives marginally increased their share of the vote province-wide — should be of concern for the party. Failing to hold the seat would not bode well for Scheer's potential to make further gains for the Conservatives in the province.

Ouellet not planning to try for federal seat

The Bloc Québécois once dominated the Saguenay–Lac-Saint-Jean, a traditional heartland of the sovereignty movement. But the party was shut out in the region in 2011 and 2015.

Lac-Saint-Jean was held by the Bloc between 1993 and 2007 and three of the region's five seats are currently held by the provincial Parti Québécois. These roots would seem to make the riding an attractive one for Ouellet, who took over the Bloc's leadership in March.

Ouellet does not hold a seat in the House of Commons but still sits in Quebec's National Assembly as an independent, representing a riding south of Montreal. The province holds its next election in the fall of 2018, and Ouellet says she will finish her term. In the meantime, she is splitting her time between Quebec City and Ottawa.

In a press conference Monday, Ouellet said she would not run in Lac-Saint-Jean, as someone from the region should run for the Bloc instead.

Other considerations, including the struggles the Bloc might have in paying their leader a salary to replace what Ouellet currently receives as an MNA, could be at play.

If Ouellet finishes her term in the National Assembly and does not run for re-election provincially, she will be owed a "transition allowance," a payout MNAs get from the province if they complete their term and either decide not to run or are defeated. As Ouellet was once a PQ minister, she receives more than other MNAs, about $125,000. She will not receive this allowance if she resigns her seat before the next provincial election.

Liberal-NDP swing in Quebec

The NDP was the runner-up in Lac-Saint-Jean in 2011 and 2015, but polls in Quebec have shown a significant swing away from the New Democrats and towards the Liberals.

The byelection will serve as a test of these shifting opinions in Quebec. The Liberals are counting on making gains in Quebec in 2019 to offset potential losses elsewhere, such as in Ontario and Atlantic Canada, while the NDP is hoping to hold its seats in Quebec, which represents over a third of the party's national caucus.

Additionally, if the byelection is held after the NDP selects its new leader in October, it could be a baptism of fire for whoever they choose.

Four-way toss-up?

All four parties would have reasonable hopes of winning Lac-Saint-Jean. Applying the difference between the 2015 election and the latest polls in Quebec would drop the Conservatives down to between 26 and 34 per cent in Lac-Saint-Jean, followed closely by the Liberals at 20 to 26 per cent, the NDP at 15 to 26 per cent and the Bloc at 16 to 25 per cent.

While that still puts the Conservatives (narrowly) at the top of the heap, this does not account for the hit the party might take from Lebel's departure. Instead, this crude estimate suggests any of the four parties could win Lac-Saint-Jean with less than a third of the vote.

And that wouldn't be unusual for the region. In 2015, the NDP won Jonquière with just 29.2 per cent of the vote and the Liberals took Chicoutimi–Le Fjord with 31.1 per cent.

So one of two (or potentially three) new leaders will hope that the byelection in Lac-Saint-Jean will provide signs to their respective parties that they can lead them to the promised land in 2019, while the Liberals will be looking for evidence they can make good on their gains in the polls in Quebec.

As byelections go, Lac-Saint-Jean could prove to be a very revealing one.