Liberal MPs and leadership hopefuls Joyce Murray and Marc Garneau didn't let up in their efforts to challenge frontrunner Justin Trudeau during the fourth of five debates in Halifax on Sunday, as all eight of the Liberal leadership hopefuls made a last-ditch effort to nab new supporters before tonight's midnight deadline.

But first, it was Trudeau's turn to take aim with Garneau's leadership campaign, telling his fellow MP from Quebec he was "incredibly proud of the high-level positive campaign" he's been running which is the least he could say about "the top-down, backroom-heavy, negative campaign that has been run by other people in this campaign."

"I think there is a message there," Garneau joked.

"We have to choose the right leader. I believe that Canadians want to see substance, they don't want empty words," Garneau fired back.

Trudeau defended his leadership record countering back that that he has been "just as specific as everybody else" in this campaign saying "I'm proud to have taken positions against the Northern Gateway [pipeline], in favour of foreign investment, in favour of the legalization of marijuana…"

Murray, the Liberal MP from Vancouver, asked Trudeau – just as she did in the first debate – what his plan was for defeating Stephen Harper's Conservatives in 2015.

Trudeau complimented Murray for running a positive campaign but took aim with her strategy of political cooperation.

"If you make a deal with the NDP, that positive approach is the first thing to go out the window. We have a federal NDP that is playing dangerous games around separatism by pandering to Quebec," Trudeau said.

Murray shot back, "if you were actually listening to Canadians, you would know that two-thirds of Canadians want proportional representation so that their vote counts…and over half of Liberals themselves, if you were listening, want us to do a one-time co-operation so we don't split the vote."

The problem with proportional representation is that it "actually increases partisanship," Trudeau said. "What we need is a preferential ballot."

Murray said a preferential ballot system was "business as usual."

Garneau also challenged Murray's plan for one-time political co-operation.

"How will you deal with the reality? Mr. Mulcair has said 'it'll be over my dead body that I'll work with the Liberal Party.' He wants to crush us," Garneau said.

Murray explained that her plan for one-time co-operation with the New Democrats and Greens was like "a Canadian hockey team coming together in the Olympics in 2010 and winning gold, then going back and competing with each other once again."

Eight Liberal leadership candidates are left vying for the party's top job after George Takach, the Toronto lawyer who advocated for a digital economy, withdrew from the race on Monday to throw his support behind Trudeau.

The eight candidates are MPs Marc Garneau, Justin Trudeau, and Joyce Murray; former MPs Martha Hall Findlay and Martin Cauchon; lawyers Deborah Coyne and David Bertschi; and retired Lt.-Col. Karen McCrimmon.

Cauchon took a jab at Garneau for calling on Trudeau to engage in a one-on-one debate at the exclusion of the others.

"It is great to be part of a one-one-one [debate] which is non-exclusive bringing all the candidates involved," Cauchon said.

The former cabinet minister from Quebec told CBC News on Saturday that Garneau went "too far" in calling for a one-on-one with Trudeau saying it showed "a lack of respect" towards the other candidates.

Trudeau refused to accept Garneau's challenge, tweeting minutes after the former astronaut had finished speaking, "I respect all the candidates for #LPCLdr. See you in Halifax, Marc [Garneau]. I hear there are 1 on 1s. ;-)".

Garneau told reporters, during a press conference in Ottawa on Monday, that Trudeau was "untested" and was hiding behind a "carefully crafted public relations campaign."

Last chance to nab supporters

On Sunday, Scott Armstrong, the Conservative MP from Nova Scotia, issued a press release calling on all the Liberal contenders to make their economic policies clear.

"Why can they suggest grandiose national schemes, but not even discuss how or when to balance the budget?"

"Is it because they have no solutions to offer – not their frontrunner, whose own competitors acknowledge lacks experience – and not anyone else...?," Armstrong said.

During his closing statement, Trudeau said "the attacks have started" because the Liberal party has been successful at drawing interests.

"Stephen Harper is afraid of engaged, empowered, active citizens. Stephen Harper's not afraid of me, he's too busy being afraid of you," Trudeau said elliciting a loud cheer from the crowd of supporters.

After the fourth debate, Garneau continued to position himself as the candidate with the most leadership experience.

Garneau said, during his closing statement, "leadership isn't an entry level position… we need in our wisdom, a leader who is ready to take on that task now. Not later, some time in the future."

Murray presented herself as the candidate best placed to bring renewal to a party now in third place.

The businesswoman and Vancouver MP said her plan for one-time political cooperation was bringing some Liberals who had not voted in years back to the party as well as "attracting people who have not been in the party before but want to see Stephen Harper defeated."

Murray made it clear that she is not in favour of a merger with the Opposition New Democrats.

While Findlay was paired with Trudeau for a debate in a group of three, she did not challenge him with the same fervour she did in the third debate in Mississauga.

The former MP drew boos from the crowd of supporters when she suggested Trudeau's privileged background rendered him out of touch with the middle class, a comment she later apologized for.

The format of Sunday's debate included both one-on-one exchanges and debates among groups of three.

Topics included regional development, women's issues, agriculture and rural affairs, health care, job creation, defence, and foreign policy. Candidates were also be given time to ask questions on any topic of their choosing.

The debate moderator was Cornwall city councillor and former Liberal candidate Bernadette Clément.

While Sunday is the deadline for candidates to enlist new supporters, Liberal party members and supporters will have to register by March 14 in order to vote for a new leader beginning April 7.

The fifth and last debate will be held in Montreal on March 23 with a new party leader being announced in Ottawa on April 14.