Seven federal New Democratic Party leadership hopefuls squared off against one another for the last time before the party's convention in two weeks, with perceived frontrunner Thomas Mulcair again coming under fire at today's debate in Vancouver.

The subject of the sixth and final debate, "Opportunities for Youth and New Canadians," was moderated by Barbara Yaffe, columnist with the Vancouver Sun.

Yaffe referred to former leader Jack Layton's last letter to Canadians in which he spoke to young Canadians: "I believe in you. Your energy, your vision, your passion for justice are exactly what this country needs today," Layton wrote before his death on Aug. 22.

NDP leadership candidates Thomas Mulcair, from left, Brian Topp, and Nathan Cullen pose for a photograph prior the party's final leadership debate in Vancouver on Sunday. (Darryl Dyck/Canadian Press)

Yaffe asked the candidates what they would do to help Canada's youth.

Mulcair, a Quebec MP, said it is important for the NDP to reach out to Canada's youth because 65 per cent of young people between the ages of 18 and 25 did not vote in the last federal election.

Manitoba MP Niki Ashton, 29, said she was proud to come from what she called "the Jack Layton generation. A generation that was inspired to think about politics differently, to think about leadership differently."

British Columbia MP Nathan Cullen, in turn called for the party to consider proposing lowering the voting age to 16.

Mulcair under fire

During the first round of question period-style questions, Ashton, Peggy Nash, Brian Topp and Paul Dewar all directed their question at Mulcair. At the heart of their questions was Mulcair's plan to modernize the party and reach beyond a traditional social democratic base.

"What specifically did we run on in 2011 that you would change?" Nash asked Mulcair, while Topp asked the Quebec MP, "instead of saying our party's the problem, shouldn't we be attacking issues our party was founded to fight?"

Ashton asked, "Would it not make more sense to go after Stephen Harper's policies than criticize our own party?"; Dewar asked, "How can you inspire people to vote for our party when you don't seem to be inspired by our party, how do you do that?

Mulcair defended his vision for modernizing the party's "boiler-plate language" and "adapting" the message to local campaigns: "What we did in Quebec is to reach out beyond our traditional base, connect with people who shared our values and our goals but had never worked or voted for us in the past."

What the NDP accomplished in Quebec, he said, could work in the rest of Canada.

"There is no contradiction between the campaign we ran in Quebec and the campaign we ran in the rest of Canada," Mulcair told journalists after the debate.

According to Topp, Mulcair's vision would move the party over to the centre. "I think it's wrong because if there are two Liberal parties in front of the people of Canada at the next election, they'll vote for the real one. So we're condemned to be ourselves," Topp said.

Dewar, Nash and Ashton have all to some extent argued the party needs to stick to its core principles.

And in speaking with reporters after the debate, Topp reiterated his proposal that "the party build on the mainstream of the progressive left, and that the NDP take on the Conservatives on the heart of the agenda."

For his part, Cullen said he rejected the idea that Mulcair's vision would betray NDP principles. "I think it's wedge politics but done within the family," he said.

Cullen's plan to hold joint nomination meetings with the Liberals and the Greens in ridings currently held by the Conservatives has also come under considerable opposition from party rivals.

But during the debate, Cullen pointed out he wasn't the first New Democrat to have that idea: "You know who first proposed the idea and did run joint nomination meetings? It was that heretic Tommy Douglas."

Michael Prince, a professor at the University of Victoria, said members are asking themselves whether they should support candidates who want the party to stick to its core social democratic principles, or get behind those who want to modernize it to attract more voters.

Tensions among rivals

A testy exchange came when Cullen asked Martin Singh, the unelected businessman from Nova Scotia, to apologize for calling Topp a liar in last week's Montreal debate.

Not only did Singh not apologize, he defended his previous comments, saying: "I will not be bullied into backing down from the truth because of false accusations about me co-ordinating debates with our candidates. Make no mistake, I'm in this campaign to win and my campaign strategies are my own."

NDP leadership candidate Peggy Nash laughs as she poses for photographs after the debate Sunday. The party elects a new leader to replace the late Jack Layton at a convention on March 23 and 24 in Toronto. (Darryl Dyck/Canadian Press)

Allegations of complicity between the Singh and Mulcair camps surfaced after the Montreal debate, when Singh attacked Topp and Nash, two of Mulcair's top rivals. Mulcair has categorically denied any co-ordination with Singh.

After the debate, Cullen called Singh's accusations against Topp "outrageous and offensive," adding New Democrats don't like the tone Singh took with Topp, whom he calls both "a friend and a proud, long-serving New Democrat."

Cullen said he had expected that Singh would apologize, but when Singh refused to do so Cullen thought it was best not to give Singh any more air time.

Singh did not immediately make himself available to reporters after the debate.

Electing a new leader

Nash, who has argued in favour of electoral reform and bringing in a system of proportional representation, made a bold prediction after the debate. "I think that the way this is shaping up is Tom [Mulcair] and I on the last ballot," she told reporters.

But the party's preferential ballot system makes it impossible to say whether any candidate has done enough to win the leadership.

New Democrats are currently in the process of mailing in their ballots or voting online. Those who attend the leadership convention in Toronto — about 3,000 are expected — will be able to vote there as well.

British Columbia will play a big role in electing a new party leader. The province is home to roughly one-third of the party's 128,351 members, the largest such group in the country.

The new leader will be announced at the Metro Toronto Convention Centre on March 24.