Toronto's Kensington Market is always awash in colour, from the produce piled high on wooden stands to the World Cup jerseys swinging on lines stretched across storefronts.

But the most prevalent colours on display these days are the orange and red campaign signs of the NDP and Liberal candidates competing to succeed Olivia Chow as the MP for Trinity-Spadina.

There's no blue to be seen here. The Conservatives aren't a factor in Monday's byelection, one of four called by Stephen Harper for the day before Canada Day.

It's a battle between the two opposition parties for so-called progressive voters, the first of what promises to be many similar contests in urban ridings across Canada in next year's federal election.

"I actually believe that urban voters are becoming much more sophisticated," said Toronto Coun. Kristyn Wong-Tam, who represents a downtown ward that's adjacent to the riding.

"They are looking for the best ideas and the most innovative solutions. And I don't believe any one single party has a monopoly on that.''

Trinity-Spadina is a perfect example of the challenges faced by the NDP and Liberals.

Thousands of potential voters live in expensive new condo towers down by Lake Ontario. Still more live in ethnic neighbourhoods where, Italian, Portuguese and Mandarin are the dominant languages. There are areas of great wealth, as well as people living on fixed incomes.

It's also a swing riding — alternating between the Liberals and the NDP for the last four decades — where the results are usually a reliable indicator of which party will get elected.

Since 1972, every time the Liberals have won the riding in a general election, they've gone on to form the government.

Each time the NDP has won, the Conservatives have taken power.

That makes the stakes unusually high for a byelection.

Leaders highly visible

NDP Leader Tom Mulcair has been in the riding four times, including last weekend, stumping with candidate Joe Cressy.

NDP Leader Tom Mulcair campaigns in Trinity-Spadina

NDP Leader Tom Mulcair, speaking with the CBC's Chris Hall, has visited Toronto's Trinity-Spadina riding to campaign four times. (Michelle Gagnon/CBC)

Liberal Leader Justin Trudeau has made about the same number of visits for his candidate, Adam Vaughan.

Both campaigns are hitting on similar issues: the need for a national housing strategy, more money for child care and public transit.

So the leaders keep coming back, to highlight their differences.

Mulcair's pitch to voters is that the NDP has the best track record in promoting an urban agenda in Ottawa, and only his party can be trusted to deliver.

"You see, the Liberals have a record of promising things that they know people want to hear, but then doing whatever they want in power, and it's usually the opposite of what they promise,'' he said during a chat outside a Kensington Market coffee shop.

"We're not going to talk about urban issues during the campaign and then forget about them months after we're elected like the Liberals did. We're actually going to do it.''

Trudeau shrugs off the criticism. He's not interested in re-hashing the past, he's looking to the future.

At a restaurant in a predominantly Portuguese-speaking neighbourhood, Trudeau arrives with Vaughan. Both are wearing Portugal World Cup scarves as they wade into a lunch-time crowd to shake hands.

"What I'm offering is an extraordinary team with a robust and responsible plan to govern this country, bringing together great people from across the country. And I'm going to happily stack up the Liberal team, the Liberal platform, against the NDP when the time comes, but it will be because I'm focused on how to govern this country right.''

Justin Trudeau campaigns with Adam Vaughan

Liberal Leader Justin Trudeau, centre, and Liberal candidate Adam Vaughan, left, wear Portuguese scarves as they campaign in Trinity-Spadina with Nova Scotia MP Scott Brison. (Michelle Gagnon/CBC)

In 2011, the NDP captured a huge swath of urban ridings from the Liberals, most notably in Toronto and Montreal. The surge also played a role in helping the Conservatives take some urban seats away from the Liberals.

For some urban activists, a resurgent Liberal Party under Trudeau suggests more vote-splitting between the NDP and Liberals — making it potentially more difficult to topple Harper's government.

That's not a universal view.

Alice Funke, who maintains the online data base called the Pundits Guide to Canadian Federal Elections, said the Liberal/NDP battle for core urban seats is interesting, but less significant to the Conservatives' fortunes in 2015 than what she calls a ‘'pincer effect'' of seeing a Liberal resurgence in the more affluent ridings north of Toronto and greater NDP strength in working-class ridings in the West.

Either way, TrinitySpadina is a significant test for the two opposition parties. Voter turnout at advance polls has been strong, as the NDP and Liberals work to pull votes from street to street, poll to poll, high-rise to high-rise and position themselves for the battles to come.