Voters in four ridings spread across the country go to the polls Monday in federal byelections that will provide a midterm report card on Justin Trudeau's Liberal government and present another tough electoral test for Conservative Leader Andrew Scheer.

Two of the ridings holding byelections — in Ontario and Newfoundland and Labrador — were last held by Liberals. The other two ridings, in Saskatchewan and British Columbia, were won by the Conservatives in 2015.

The CBC's polls analyst Eric Grenier breaks down the races, and looks ahead to Tuesday's special election in Alabama. 4:46

The byelection contests are happening in:

Bonavista–Burin–Trinity in Newfoundland, last held by Liberal Judy Foote.

Scarborough–Agincourt in Ontario, last held by Liberal Arnold Chan.

Battlefords–Lloydminster in Saskatchewan, last held by Conservative Gerry Ritz.

South Surrey–White Rock in B.C., last held by Conservative Dianne Watts.

The Liberals are expected to hold onto the Ontario and Newfoundland and Labrador seats, while the Conservative seat in rural Saskatchewan is also a stronghold for the incumbent party.

But the B.C. riding of South Surrey–White Rock will be hotly contested between the two parties — putting Scheer at risk of suffering a second byelection loss as leader.

As the New Democrats did not put up strong results in 2015 in any of the four ridings, they are unlikely to play a significant role. Nevertheless, NDP Leader Jagmeet Singh will be assessed in part by how his party performs in these first byelection campaigns to be called after he became leader (two earlier contests occurred just weeks after he won the party's leadership on Oct. 1).

The outcome of the four byelections will not have any major impact on the next general election to be held in 2019. But there are nevertheless some things to watch that could provide clues as to how the parties are doing with the electorate today — and what that might mean for 2019.

Can the Liberals pull off another win?

Both Trudeau and Scheer have visited South Surrey–White Rock twice, highlighting the importance both leaders are placing on this suburban seat southeast of Vancouver.

Dianne Watts, who resigned as a Conservative MP to run for the leadership of the B.C. Liberal Party, won the riding by just 2.5 points in 2015. But despite the close margin two years ago, South Surrey–White Rock has been a stronghold for the Conservatives and its predecessor parties since the 1970s.

This time, the Conservatives are putting forward former MP Kerry-Lynne Findlay, who represented an adjacent riding before her defeat in 2015.

Prime Minister Justin Trudeau, left, and South Surrey-White Rock Liberal byelection candidate Gordie Hogg attend a rally in Surrey, B.C., on Saturday December 2, 2017. A federal byelection will be held Dec. 11 for the seat vacated by former Conservative MP Dianne Watts, who stepped down to run for the leadership of the B.C. Liberal Party. (Darryl Dyck/The Canadian Press)

The byelection will test the Liberals' prospects in British Columbia, one of just two provinces — along with Quebec — in which the party has more support today than it did on election night in 2015. The Liberals have put forward a high-profile candidate in Gordie Hogg, who represented the provincial riding of Surrey–White Rock as a B.C. Liberal MLA for 20 years.

Polls suggest support in B.C. has swung enough between the Liberals and Conservatives to overturn the 2.5-point margin from two years ago. Another loss here for Scheer would be the second under his leadership after his party failed to hold the formerly Conservative seat of Lac-Saint-Jean, Que. in an October byelection.

How Scheer, Singh appeal to suburban voters

Like South Surrey–White Rock, the contest in the Toronto riding of Scarborough–Agincourt will also gauge the suburban vote.

The Liberals' Arnold Chan won the seat, a very safe Liberal riding in the 1990s and 2000s, by 14 points in 2015. Chan died earlier this year and his widow, Jean Yip, is the Liberal candidate.

Toronto was swept by the Liberals in the last election and the party is looking to repeat the feat in 2019. The Conservatives might be targeting seats further afield in the Greater Toronto Area, but Scarborough–Agincourt was one of the rare ridings in which the Conservative vote actually went up in 2015.

If the Conservatives' Dasong Zou holds those gains from the last election or improves upon them it would suggest the party could win back some of the seats it lost two years ago.

South Surrey-White Rock Conservative byelection candidate Kerry-Lynne Findlay, front, sits with Conservative Leader Andrew Scheer during an interview at a cafe, while campaigning in Surrey, B.C., on Monday December 4, 2017. A federal byelection will be held Dec. 11 for the seat vacated by former Conservative MP Dianne Watts, who stepped down to run for the leadership of the B.C. Liberal Party. (Darryl Dyck/Canadian Press)

The New Democrats have little at stake in the riding, where they took just 7.9 per cent of the vote last time. But Singh was born in Scarborough and the party is banking on him to win it new supporters in the GTA, where he previously held a seat in the provincial legislature.

Though the NDP traditionally struggles in byelections in which the Liberals and Conservatives are vying for the win, a worse showing here than in 2015 could raise some questions about how Singh is resonating — and disappoint Conservatives who are hoping that the NDP will draw support away from the Liberals in the region.

Do incumbents hold their vote in strongholds?

The Newfoundland and Labrador riding of Bonavista–Burin–Trinity was won by the widest margin in the country in 2015 when Judy Foote secured it for the Liberals by 72 points. Churence Rogers, a local mayor and the Liberals' candidate, might have a tough time matching that score.

The only question is just how much of the Liberal vote turns out and whether the party can maintain the sky-high support it had in the riding in 2015. The Conservatives' Mike Windsor will be looking to beat the 10.1 per cent score he put up two years ago.

It is a similar story in Battlefords–Lloydminster, which now-retired Conservative MP Gerry Ritz won by 43 points in 2015, and routinely won by 35 points or more over the last 20 years. Rosemarie Falk will carry the Conservative banner this time.

The party has tended to do well in strongholds in byelections, increasing its support by seven to 11 points in three Alberta ridings it won in landslides in 2015. Falk's score will be held up against that standard.

The NDP's Matt Fedler will also be looking to hold on to the second place his party has secured in every election in Battlefords–Lloydminster since 1997. Of the four ridings being contested on Monday, this is the one where NDP support was highest in 2015.

But all eyes will be on the results in South Surrey–White Rock.

If the Liberals win in B.C., Justin Trudeau's party will have made a net gain of four seats in byelections since 2013 — a number not seen for that party under any leader since Mackenzie King.

The last leader, however, to go down two seats in byelections was Stéphane Dion, a few months before he led the Liberal Party to defeat in 2008's federal election. Scheer will have to hold on to South Surrey–White Rock to avoid matching that record.