NDP Leader Tom Mulcair and Liberal Leader Justin Trudeau can't stop talking about the middle class. In ads, speeches and in question period, the two party leaders refer repeatedly to middle-class families.

The idea of the middle class is also a major theme in the byelection race in Toronto Centre, one of four ridings seeing voters go to the polls Monday. Bourassa, in Montreal, and Manitoba's Provencher and Brandon-Souris are also seeing hard-fought races.

In a recent ad, Trudeau refers to the middle class as his priority.

"Because my priority is the Canadians who built this country: the middle class, not the political class," he said.

Mulcair also referred to the middle class in his speech to caucus at last spring's policy convention.

"Today, our country faces levels of income inequality not seen since the Great Depression, and the middle class is struggling like never before," he said in April.

"Middle-class wages are consistently on the decline. Yet the Conservative solution is to demand even more from you and to leave even less to our children and our grandchildren."

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.

Despite that, it isn't all that easy getting politicians to define what they mean by the term.

"I think that the key thing to consider when we're talking about the middle class is the self-definition and our expectations around being in the middle class," said Chrystia Freeland, the Liberal candidate in the Toronto Centre byelection and author of Plutocrats.

"I think that that sort of combination of working hard, expecting that your own hard work will translate into a comfortable and secure lifestyle for you, and then a belief that if your children work hard and get a good education" they'll do well, she said.

93% say they're middle class

Nathan Rotman, national director of the NDP, said the middle class isn't easily definable.

"Certainly I think it's people who are struggling to make ends meet today," he said.

"People are saying well I hear that the economy is doing well, but I don't feel that it's working for me. I don't see the fruits of that strong economy in my day-to-day life... Certainly [people who are] not in the top one per cent and certainly not in the bottom five."

In fact, a specific definition of the middle class might be the 60 per cent of Canadians who make more than the bottom 20 per cent (more than about $10,000 individual income) and less than the top 20 per cent (less than about $60,000), 

Average annual individual income in Canada in 2011 was $39,300, with $29,000 clocking in as median income, the income around which half of households earn more and half of households earn less.

Average family income in Canada in 2011 was $66,200 and median family income was $48,300.

But the vast majority of Canadians identify as middle class, despite there being specific numbers to help people figure out where they fall.

That's an enormous pool of voters to try to attract. 

"Between lower middle class, middle class and upper middle class, you have 93 per cent of Canadians self-identifying," said Tony Coulson, group vice-president of corporate and public affairs at Environics Research. 

Parties direct policies to attract middle class

It's no wonder the Liberals and NDP want those middle-class votes. When politicians directly address the middle class, they're addressing almost everyone. 

Coulson says people may feel they're being reached out to when a politician says they're speaking to the middle class.

"When they hear, you know, I'm working for the middle class, I'm concerned about the middle class, those kinds of statements, they likely will feel some, some affinity toward that," he said.

The NDP and Liberals have both focused policies on appealing to the middle class, including the New Democrats' emphasis on household debt and lowering credit card fees, and the Liberals' references to making ends meet and affordable education.

One Conservative strategist says it's not only the opposition that's trying to appeal to those voters — it's just that the government does it more subtly.

Chrystia Freeland and Linda McQuaig rivals in Toronto Centre

Liberal candidate Chrystia Freeland, left, and NDP candidate Linda McQuaig are facing off over the middle class in Toronto Centre. (Handout photos)

"It strikes me that it might detract from your credibility a little bit if you put it out there publicly that you are targeting a major demographic group like the middle class. It sounds tactical and it sounds kind of manipulative," said Yaroslav Baran, a principal at Earnscliffe Strategies.

"You'll see the Conservatives talking about families, families with children, consumers, taxpayers, seniors, seniors on fixed income, our brave men and women in uniform, etc. These are groups that it's easier for people to identify with or identify themselves as being a part of. They're more specific communities."

Coulson agrees that the strategy may not be targeted enough, since there are so many sub-types within the group of Canadians who call themselves middle class.

"Young urbanites, multicultural suburban households, aging folks in the inner suburbs and older people in small towns and rural areas," are all included, he said.

"So it's really a diverse group that falls within middle-class definitions and as a result the tactical outreach might need to be more tailored to those groups."

Byelections a rehearsal for 2015

NDP candidate Linda McQuaig has written several books about income inequality, including last year's The Trouble with Billionaires. Because so many people identify as middle class, she says it's better to look at income inequality as a measure. 

"In the last 30 years, virtually all the income gains have gone to the top. Particularly to the top 10 per cent, particularly the top one per cent. And when you get below that, what you find is incomes have actually stagnated or even declined," McQuaig said.

The four byelections on Monday allow the parties to rehearse their general election campaign messages.

Toronto Centre is particularly interesting with Freeland and McQuaig, two middle-class champion heavyweights, facing off.

It's so far led to fights over which candidate bought a more expensive home and has worked harder to keep professional jobs in Canada.

As Canada's politicians refine their messaging in the two years leading up to the next federal election, the focus on "middle-class priorities" is bound to get sharper.

"I think the middle class is one that encompasses most people and certainly is around, that talks to, to both the struggles that people have and how they see themselves, and I think that's an important way of reaching out and engaging voters," Rotman said.