Voters head to the polls in four ridings Monday to cast their ballots in byelections that traditionally get little attention, but have taken on greater significance as tests of the leadership of Justin Trudeau and Tom Mulcair, as well as the scope of the fallout from the ongoing Senate expenses scandal.

Here's a look at the four ridings, the leading candidates, the big stories from the campaign trail and what the pundits predict will happen once the polls close.

Brandon-Souris:

This is one of two Manitoba ridings looking for a new MP, and is the one of the four that has the best chance in ending in an upset. The long-time Conservative stronghold -- it has only gone Liberal once in the last 60 years -- might just be ready for change, if the polling numbers can be believed. Some polls have given the Liberals as much as an eight-point lead and, sensing an opportunity, Leader Justin Trudeau has made two trips to the riding to get out the vote.

It should be noted that the Liberal candidate finished fourth in the 2011 federal election.

"They're taking the poll results as an indication that there may be a seat to steal," Winnipeg Free Press columnist Dan Lett told CTV's Question Period on Sunday. "And it would be a significant expression of Conservative dissatisfaction in rural Manitoba if they were to win this seat."

Alice Funke of PunditsGuide.ca says until recently, Brandon-Souris would have represented an easy win for the Conservative government. However, the ongoing Senate expenses scandal has left some Tory voters disillusioned, as did the Tory candidate nomination process. Former MLA Larry Maguire was acclaimed after two candidates were disqualified over issues with their paperwork.

And so, the Conservatives have cast aside a general aversion to campaigning in byelections and made moves to stave off a Liberal advance. Last week, a letter signed by Harper began appearing in mailboxes across the riding, warning voters of Trudeau's support for legalizing marijuana and his opposition to mandatory prison sentences.

The letter also touted Maguire's credentials in contrast to Liberal candidate Rolf Dinsdale who, the letter states, "only moved back a few months ago to run in this by-election."

A flyer circulating through the riding also mocked Dinsdale for performing in a band called "S-T from Hell," and included a somewhat familiar slogan: "He only came back to get elected."

But Dinsdale, Funke notes, is a strong candidate for the Liberals with long-time family ties to the riding - and to the old Progressive Conservatives. His father, Walter, was a PC MP for more than 30 years.

"If this was a frontrunner's campaign and (the Conservatives) were feeling it like frontrunners, they wouldn't be taking such dramatic moves in the final days of the campaign," Funke told CTV's Power Play on Friday.

NDP Leader Tom Mulcair has also visited the riding, saying he believes that there's a three-way race.

Provencher:

Unlike Brandon-Souris, Provencher is very likely to remain Tory blue Monday, despite a late controversy last week involving Conservative candidate Ted Falk.

After a gay teen was taunted by students during a television interview with CBC, Falk was quoted by a local newspaper as questioning whether the taunting was staged.

"Whether that was staged we don't know," Falk said.

The teen, Evan Wiens, told the CBC he was hurt and confused by Falk's words. In turn, Falk said he is "100 per cent against bullying of anyone," but again wondered if the taunting was staged.

Lett called the Provencher byelection "a bit of a gong show," but said the seat is unlikely to change hands as a result.

"In the end I don't think the Liberals or New Democrats in that part of Manitoba really have a whole lot of interest in showing up to vote because it's too steep a hill to climb," Lett told Question Period.

Former justice minister Vic Toews won the riding in the 2011 election with nearly three times the votes of the NDP candidate.

Bourassa:

While Bourassa is considered a safe Liberal riding, the New Democrats are fielding a star candidate in former Bran Van 3000 lead singer Stephane Moraille, who is now a high-profile intellectual property lawyer and well-known in the local Haitian community.

Liberal candidate Emannuel Dubourg resigned from the Quebec National Assembly to run for the federal seat, which was held by Denis Coderre until he stepped down to run, successfully, for mayor of Montreal.

CTV political commentator Jean Lapierre noted that Coderre won the riding by more than 3,000 votes in the last federal election, so the seat is "the Liberals' to lose."

However, he told Question Period that the NDP has been "very, very aggressive," with Mulcair visiting the riding to glad-hand voters alongside Moraille.

However, voter turnout is expected to be quite low because Montreal just came off its civic election and there are also provincial byelections coming up, "so people are all mixed up," Lapierre said.

Funke agreed that the seat should stay in Liberal hands, despite claims by some pundits that "it's closer than we think."

"That campaign has really been occluded by the municipal election, so they've really only paid attention for the last week-and-a-half or so," Funke told Power Play.

Unlike their push in Brandon-Souris, the Conservatives have not made a big campaign push in Bourassa, a reflection of the fact that the Tories have little hope there.

"In Bourassa, he'll probably make something like 10 per cent of the vote," Lapierre said.

Toronto-Centre:

The battle for Bob Rae's old seat has perhaps produced one of the more interesting campaigns, with two high-profile journalists going head-to-head over who best represents middle-class values.

While the riding is a Liberal stronghold, Monday's byelection is shaping up to be a tight, two-way race between Liberal candidate Chrystia Freeland, who has returned home from New York to run for public office, and NDP candidate Linda McQuaig.

Both candidates have written books about the declining middle class and have taken each other to task over their differing views on the issue in what Funke calls "high-quality debates."

But it's been difficult to get a sense of who exactly has the edge, she says, because voters in the downtown Toronto riding have fewer landlines for pollsters to call.

"So it's really being fought on the streets, one high-rise at a time," Funke said of the campaign.

John Moore, host of "Moore in the Morning" on Newstalk 1010, points out that over the last handful of elections, the Liberals beat the NDP by margins of about 3-1. However, in 2011, he told Question Period, the Liberals edged the NDP 40 per cent to 30 per cent.

"Chrystia Freeland, I think a lot of people see her as a kind of Michael Ignatieff candidate, she's very smart, she's very internationally known, she's done TED talks," Moore said. "But Linda McQuaig really seems to have the retail aspect of this down and I think that there's the possibility either it's going to be very close, or the NDP could steal it."

Both Trudeau and Mulcair have made several appearances in the riding, doing everything from shaking hands with commuters at subway stations to appearing at high-profile events, a clear sign each believes his party could come away with a victory.