Voters in three provinces are casting ballots in four federal byelections.

Monday's byelections have taken on greater significance thanks to hotly contested races between the opposition NDP and Liberals in Ontario and Quebec and the controversy over the expenses scandal in the Senate.

Here are four things to know about the races as voters go to the polls.

1. Tory split raises Liberal hopes in Brandon-Souris

This byelection should have been a shoo-in for Tory candidate Larry Maguire, a prominent provincial MLA running in a Conservative stronghold.

But a controversial nomination process disqualified two potential candidates because of incomplete paperwork.

CBC byelection coverage tonight Return to CBCNews.ca after polls close at 9:30 p.m. ET (8:30 p.m. CT) for full coverage of the results. When to vote Bourassa (Quebec): 9:30 a.m. to 9:30 p.m. ET Brandon-Souris (Manitoba): 8:30 a.m. to 8:30 p.m. CT Provencher (Manitoba): 8:30 a.m. to 8:30 p.m. CT Toronto Centre (Ontario): 9:30 a.m. to 9:30 p.m. ET More information and where to vote in your riding at Elections Canada's website.

Maguire was acclaimed and that soured many Conservatives, one of whom left the party to run as a Liberal candidate. (He didn’t win the nomination.)

Maguire has since been criticized for not attending two of the four candidates’ debates and for a series of attack ads against his Liberal opponent Rolf Dinsdale, whom Maguire paints as a parachute candidate from Toronto, even though Dinsdale's father was the Progressive Conservative MP in the riding for more than three decades.

The Senate scandal in Ottawa isn’t helping Maguire either.

Many voters say they’re not happy with the Harper government and want to send a message. A byelection is a safe way to do that.

The federal party leaders sense that, and have made this race a priority.

Liberal Leader Justin Trudeau has been here three times, NDP Leader Tom Mulcair has visited twice and several federal Conservative cabinet ministers have made the trek to the riding.

Kelly Saunders, a political scientist at Brandon University, says this race could be a game-changer and foreshadow what could happen in the 2015 general election.

“This is Western Canada, where the Liberals haven’t been able to build up much support. If they can win this seat in a safe Tory riding like Brandon-Souris, they will use that to build momentum. For Conservatives, they won’t be able to take things for granted as they maybe have in the past,” she says.

2. Liberals, NDP square off in Montreal

Voters in North Montreal’s diverse Bourassa riding are choosing a new MP to replace Denis Coderre, who stepped down to run for municipal office and is the city's newly elected mayor.

Both of the perceived lead candidates were born in Haiti, which is seen as an advantage in a federal district where one in every five voters identifies as Haitian.

Though the district is comprised of varied ethnic groups and socio-economic statuses, employment is the dominating priority for voters, as the average annual income in Bourassa is $26,000 and the unemployment rate exceeds 13 per cent.

To replace Coderre, the Liberals chose six-year Quebec National Assembly MLA Emmanuel Dubourg.

"We will set up a roundtable consisting of elected municipal, provincial and federal governments to address issues of youth employment, as well as safety," said Dubourg. "The district’s police commander said that the biggest problem he sees is domestic violence."

Dubourg's main opponent is the NDP’s Stéphane Moraille, a former singer in the musical group Bran Van 3000.

"The NDP wants to introduce a tax credit for hiring young people," said Moraille. "We are on a crusade against the costs of credit cards that eat away at incomes. We want to attract investors to employ people."

The Bloc Québécois candidate, Daniel Duranleau, has also tried to appeal to the Haitian community in this historically federalist riding by promoting the work of the Quebec government during Haiti’s 2010 earthquake.

"The Quebec government was very proactive and Ottawa worked at a much slower pace. In the end, those who have suffered are the people," said Duranleau.

The Conservative candidate, Mahmoud Rida, was absent during the candidates' debate.

The Green Party's candidate was former NHL player Georges Laraque, but he dropped out to address criminal fraud charges and was replaced by Danny Polifroni.

3. Media stars lead polls in Toronto Centre

The race for former interim Liberal leader Bob Rae’s riding of Toronto Centre will likely be decided by two journalists. The riding has been Liberal red for 20 years and was vacated by Rae in June.

Polling suggests Liberal candidate and former Reuters executive and author Chrystia Freeland is leading NDP candidate Linda McQuaig, an author and journalist. Veteran reporter John Deverell is running for the Green Party.

McQuaig, a former Toronto Star columnist, was visited in her riding by Mulcair on Friday.

“It’s a Liberal stronghold. We’ve known that since the beginning,” Mulcair said while campaigning at a downtown subway station. “But people want to send the message that there is an option, that they’re tired of alternating between Liberal corruption and Conservative corruption.”

Trudeau has also visited what he hopes will remain his party’s riding. Freeland has said the big issues for residents are the economy and jobs.

“People keep coming back to that,” Freeland said. “A lot of people are worried about how Canadian politics is working.”

At least one analyst believes the unfolding scandal in Ottawa might overshadow local issues at the ballot box.

“I would say the Senate scandal is now reverberating, so that’s going to have an effect,” said University of Toronto professor Nelson Wiseman. “Then the question becomes: is it going to benefit the NDP or the Liberals?"

Interest in the byelection appears high in Toronto Centre, with hundreds packing a high school gym last week. Conservative candidate Geoff Pollock told the crowd that if elected his party would “continue to create jobs, and that’s something this government has done under the leadership of Stephen Harper.”

But people here seem to be more concerned with the lack of affordable housing, public transit and federal subsidies.

“I hear constantly about the housing crisis in downtown Toronto — that is the number one issue,” McQuaig said. “People are just overwhelmed by the lack of decent housing.”

4. Conservatives appear safe in Manitoba stronghold

Manitoba's Provencher is widely expected to remain Conservative territory.

The longtime Tory stronghold was most recently represented by former cabinet minister Vic Toews, who retired from politics in July after almost 13 years as the area's MP.

The campaign leading up to Monday's vote has been relatively quiet compared to Manitoba's other byelection race in Brandon-Souris.

For the most part, Conservative candidate Ted Falk kept a low profile. He did not participate in any public debates, prompting his opponents to accuse him of hiding.

"We're busy trying to get the message out to our constituents," Falk told CBC News on Thursday, after a reporter tracked him down at a public event following numerous interview requests.

Still, polls and political observers, as well as many voters in the area, suggest he's the one to beat.

"Most of the area is Conservative-minded," said Frank Toews, a resident of Steinbach, Man.

But Falk caused a stir at the end of the campaign when he suggested in a newspaper interview that an openly gay teenager in Steinbach may have staged his own bullying incident earlier this year for a CBC News report about Manitoba's anti-bullying legislation.

Evan Wiens, 17, said he was hurt by Falk's suggestion and called on him to apologize.

Falk has stood by his questioning, but said he is "100 per cent against bullying."

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