Byelections rarely have the potential for much drama. Monday's vote in the Quebec riding of Lac-Saint-Jean does.

Four parties have the potential to win the seat that former Conservative MP Denis Lebel took by a margin of just five points in 2015. The Conservatives and the New Democrats are contesting the riding with new leaders at the helm in a province that could play an important role for both parties in 2019.

The byelection will also serve as a test of the Liberals' strength in Quebec — a linchpin for them in retaining their majority government in two years.

And for the Bloc Québécois, with a recently installed new leader as well and just two seats short of official party status in the House of Commons, the results might shed some light on its future.

It isn't the only byelection being held on Monday. Voters in the Alberta riding of Sturgeon River–Parkland are also being called to the polls.

Rona Ambrose, who served as the Conservative Party's interim leader until Andrew Scheer was given the permanent job in May, won Sturgeon River–Parkland with 70.2 per cent of the vote in 2015, 55 points ahead of her nearest competitor. The Conservatives are expected to continue their unbroken winning streak in this part of the country, a streak that began in 1958.

Lac-Saint-Jean swung to the Conservatives more recently. Lebel took it from the Bloc in a 2007 byelection and he was subsequently re-elected three times.

But while the Conservatives' vote share in Quebec increased in the 2015 federal election — an increase Scheer would like to repeat in 2019 — Lebel's support dropped to 33.3 per cent. The NDP's Gisèle Dallaire finished a close second with 28.5 per cent, followed by the Liberal and Bloc candidates at 18.4 per cent apiece.

Liberals have slipped in byelections so far

To win the seat away from the Conservatives, however, the other parties will have to improve on their byelection performances so far in this parliamentary session.

Both the Liberals and New Democrats have lost support in five of the six byelections that have been held since the last general election. The Bloc picked up only 0.2 points in its only byelection test in Saint-Laurent. The Conservatives, meanwhile, have slipped in just one.

On average, the Conservatives have picked up 3.7 percentage points in byelections, compared to a drop of 2.6 points for the Liberals and 3.3 points for the NDP. A continuation of those trends on Monday would keep the seat in the Conservatives' hands.

Gains for the Liberals and Bloc in Quebec?

But Lac-Saint-Jean is only the second byelection to occur in Quebec, and it is in a much more competitive setting than the April vote in Saint-Laurent.

Quebec has experienced a big shift in voting intentions since the 2015 election, primarily to the detriment of the NDP.

The CBC Poll Tracker, an aggregation of polling data, puts the party at 15 per cent support in Quebec, down significantly from the 25 per cent the party captured two years ago.

Liberal support has slid from a high of around 43 per cent at the end of the summer, but it is still more than three points above its 2015 election performance at 39 per cent. The Bloc, too, has made gains, up three points from 2015 to 22 per cent.

The Conservatives have lost a little ground, down more than a point to 15 per cent.

If the New Democrats suffer a similar drop in support in Lac-Saint-Jean as they have in Quebec as a whole, that vote will have to go somewhere — and the Liberals and Bloc may be best positioned to scoop it up.

When the candidate matters

The challenge for the NDP — represented again by Dallaire — will be to overcome those negative provincial trends. The NDP will desperately want to turn things around in the province that provided 16 of its 44 MPs in the last election, more than any other province.

NDP Leader Jagmeet Singh with NDP candidate Gisele Dallaire, left, in Alma, Que. (Jacques Boissinot/Canadian Press)

Compounding the challenge for the Conservatives and their candidate, Rémy Leclerc, is the loss of Lebel himself.

The departure of an incumbent alone can be expected to cost a party about a 10th of its vote. But when Lebel, a local mayor, ran for the Conservatives in 2007, he boosted the party's support by 23 points over its performance in the riding in the 2006 general election.

The Liberals are following the same playbook that was successful for the Conservatives 10 years ago and are hoping to get the same kind of boost.

Their candidate, Richard Hébert, is the outgoing mayor of Dolbeau-Mistassini — a town larger than Roberval, Lebel's old bastion of strength. Combined with the drop in support for the NDP in Quebec and the departure of Lebel, it puts the Liberals in a strong position.

A visit from Prime Minister Justin Trudeau to the riding late last week suggests the Liberals are taking the byelection seriously.

But the Bloc might also prove to be a contender. In Marc Maltais, the Bloc has a candidate with a good profile in the riding. He made a name for himself during a 2012 lockout at Rio Tinto Alcan and has the backing of the union of aluminum workers in Alma, the riding's biggest population centre by far.

The Greens are putting forward Yves Laporte in Lac-Saint-Jean. The party captured 1.5 per cent of the vote in the riding in 2015.

Quebec is setting up to be one of the most important battlegrounds in 2019. All four parties are hoping to get the better of their opponents in this first skirmish.