One week down, four to go and it's clear who the main federal parties are fighting over in this federal election.

It's a battle over the middle class.

Officials inside the Conservative war room say the middle class is "central" to the party's campaign.

In one speech, Liberal Leader Michael Ignatieff said "middle class" three times.

And NDP Leader Jack Layton's speeches aren't far behind.

"The middle class tend to be the ones that vote, tend to be the ones that pay the bulk of the taxes...and they're more in play than almost any other category of voter," says media consultant Barry McLoughlin, adding that as the middle class vote goes, so goes the election. 

But who is a middle class voter?

'Middle-class means almost anybody, so it's a safe thing to say'—David Hulchanski, University of Toronto

"It is a useful sort of catch-all phrase that is conveniently quite undefined," says Queen's University professor Kathleen Lahey who teaches law and specializes in tax and social policy.

It's certainly undefined by the main political parties.

The Conservatives say they don't want to put people in boxes.

The NDP say they prefer to let people self-identify.

And the Liberals say that it's easier to describe a middle class family by their priorities than by any kind of statistical analysis.

What 'middle class' are they talking about?

"I would like to see more people ask politicians who use the term 'middle class' who they are talking about specifically," says David Hulchanski, who researches urban issues at the University of Toronto.

Hulchanski says that's because many of the promises politicians make won't actually benefit the true middle class, even if politicians say that's who the pledges are aimed at.

For example, the Conservative campaign proposal of family income splitting would most benefit households who earn more than $70,000, according to most experts.

And yet Statistics Canada's latest Survey of Household Spending shows the median pre-tax household income is $58,500.

And if middle class were defined by a middle income range, that same survey puts it at between about $47,000 and $71,000 dollars per household.

Middle class not benefitting

Univerisity of Toronto economics professor and demograhics expert David Foot argues the fact that the middle class is shrinking proves most policies are not benefitting the middle class.

In fact, he says the poor have gotten poorer and the rich, richer.

"Real wages haven't changed in the last 25 years. The average Canadian not only hasn't seen any changes in their average take-home pay, but the uncertainty associated with that take-home pay has gone up dramatically," says Foot.

Lahey agrees. "It's important that people insist on not being misled by programs said to be aimed at the middle class."

But the real middle class isn't the only group being misled, according to experts.

Hulchanski says except for the very poor or the very rich, who know they are not middle class, many Canadians mistakenly think of themselves as middle class even if technically they aren't, according to their income.

He says that means they too think the promises are for them, which is convenient for politicians.

"Middle-class means almost anybody so it's a safe thing to say," says Hulchanski. "Because the listener could think that the party is speaking to them."

And voters do respond to the message, says media consultant McLoughlin.

"Most people aren't economists and they're in no danger of becoming one," says McLoughlin, explaining that most voters don't scrutinize the math. "But they do want to feel that you value their hard work and the difficulty they have putting food on the table."