New Democrat Leader Tom Mulcair and Liberal Leader Justin Trudeau continue to cast long shadows over the upcoming byelection in Trinity-Spadina, where NDP candidate Joe Cressy and Liberal candidate Adam Vaughan are seen as top contenders for Olivia Chow's vacated seat ahead of Monday’s vote.

Speaking in an interview that aired Sunday on CTV's Question Period, both Cressy and Vaughan stressed the importance of voters voting to support their respective leaders, so they could fight against Prime Minister Stephen Harper's Conservatives.

Cressy praised Mulcair as "a leader who is taking on Harper," while Vaughan said Trudeau is "doing very well" in leadership polls.

Mulcair and Trudeau have both been campaigning hard in the Trinity-Spadina riding in recent weeks, with Monday’s byelection expected to provide some indicator as to how each party might fare in the 2015 federal election.

"All of our cities need a better partner up in Ottawa," Vaughan told Question Period. He stressed a platform that focuses on city infrastructure, transit, housing and climate change, while pointing out that Trudeau is the more popular federal leader, according to recent polls.

"The message we're resonating is that it's the right party, it's the right candidate, we've got a great leader, and things are changing in Toronto," he said.

Vaughan deflected any mention of former NDP MP Olivia Chow, who surrendered her Trinity-Spadina seat to run in the Toronto mayoral race. "The real issue in this riding is not what happened last time, it's what's about to happen next time," he said.

But Cressy put the emphasis on Chow, citing her as an example of the kind of representation the NDP can provide for the riding.

"I think residents in downtown Toronto have come to expect and anticipate a certain level of strong, progressive representation," he said. "I think Olivia Chow did that very well for us, and I hope she becomes our next mayor."

Cressy pushed a platform focusing on public transit, climate change and a determination to fight Stephen Harper, particularly on the contentious Keystone XL pipeline.

"We've had consecutive Liberal and Conservative governments fail us on the big issues," he said. "What you know in this country is that you can count on New Democrats to stand up for you every day."

Cressy pointed to his track record of studying climate change, collaborating with First Nations and working with AIDS patients in southern Africa as proof of his credentials. "I've spent my life working on social justice issues at home and abroad," he said.

Cressy and Vaughan are both sons of former Toronto city councillors. Vaughan himself was a city councillor for eight years. He won the last Trinity-Spadina municipal election with about 75 per cent of the vote, but stepped down in May to pursue a seat in parliament. He got the nod in Trinity-Spadina ahead of Christine Innes, who lost to Chow in the last two federal elections.

Earlier this month, provincial Liberal candidate Han Dong unseated incumbent New Democrat MP Rosario Marchese to take the Trinity-Spadina riding in the Ontario election. Vaughan called the win a "major reversal of fortune" and said he hopes that means the riding is moving away from the NDP.

"We're looking to build on that momentum," he said. "We're very hopeful that Toronto is returning to the Liberal fold."

Mulcair and Trudeau were both in Toronto to support their candidates on the weekend before the election. Mulcair and Cressy attended a PFLAG brunch on Sunday as part of the WorldPride festivities, while Trudeau and Vaughan posed for pictures with WorldPride parade-goers downtown.

The Trinity-Spadina byelection is one of four federal byelections taking place Monday. Voters in another Toronto-area riding, Scarborough-Agincourt, will also cast ballots, as will those in the Alberta ridings of Macleod and Fort McMurray-Athabasca.