Liberal MPs and leadership hopefuls Joyce Murray and Marc Garneau didn't let up in their efforts to challenge frontrunner Justin Trudeau during the 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.

Trudeau took a shot at Garneau, when he told 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 countered back that that he has been "just as specific as everybody else" in this campaign saying "I'm proud to have taken postions agains 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 fired 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.

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.

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.

Earlier this week, Takach — who recently wrote on his website that the "last thing the Liberal Party of Canada needs is another coronation" — threw his support behind front-runner Justin Trudeau.

"Justin has the strength to lead a movement that can connect with Canadians from all walks of life, regions and communities," Takach said in a statement.

On Monday, Marc Garneau challenged Trudeau to a one-on-one debate saying his fellow contender should have the "courage of his convictions" to debate with him directly.

While he refused to blame the party for the format of the first three debates, 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."

Minutes after Garneau finished speaking, Trudeau tweeted: "I respect all the candidates for #LPCLdr. See you in Halifax, Marc [Garneau]. I hear there are 1 on 1s. ;-)".

Martin Cauchon, another candidate from Quebec, told CBC News on Saturday that Garneau went "too far" in calling for a one-on-one with Trudeau. Cauchon said it showed "a lack of respect" towards the other candidates.

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.

Retired Lt.-Col. McCrimmon and Murray, the Liberal MP from Vancouver, were the only two candidates to go one-on-one with Trudeau.

Murray was the only candidate in the first debate to challenge Trudeau saying, "if you want to replace Stephen Harper, where's your plan?"

Findlay, Trudeau and Cauchon debated each other in a group of three after the first hour of one-on-one debates.

Findlay apologized to Trudeau for suggesting his privileged background rendered him out of touch with the middle class, after the third debate in Mississauga.

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

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

Media Scenario - Halifax Debate (1) by kady23