The CBC Pollcast, hosted by CBC poll analyst Éric Grenier, explores the world of electoral politics, political polls and the trends they reveal.

Voters head to the polls in two byelections on Monday. One of them is in a safe Conservative seat in Alberta, but the other, in the Quebec riding of Lac-Saint-Jean, is being hotly contested by four parties in a province that could play a decisive role in the 2019 federal election.

So, who has the best shot of winning it?

The byelections in Sturgeon River–Parkland and Lac-Saint-Jean are being held to fill the seats vacated by former Conservative MPs Rona Ambrose and Denis Lebel.

Ambrose won her Alberta seat in a landslide in 2015. But Lebel squeaked by in Lac-Saint-Jean, defeating the New Democrats by just five points. The Liberal and Bloc candidates were not much further behind.

It is setting up to be a closely fought four-way contest — the kind of battle that could be seen throughout Quebec in 2019.

In addition to gauging all four parties' support in this key province, the byelections will also serve as the first electoral tests of Conservative Leader Andrew Scheer and NDP Leader Jagmeet Singh since they took over their respective parties.

To discuss each party's chances of winning in Lac-Saint-Jean and what's at stake, Pollcast host Éric Grenier is joined by the CBC's Nick Gamache and Radio-Canada's Louis Blouin.

Listen to the full discussion above — or subscribe to the CBC Pollcast and listen to past episodes.

Follow Éric Grenier, Louis Blouin and Nick Gamache on Twitter.