Liberal leadership hopeful Marc Garneau accused Justin Trudeau of ducking the big issues in a party leadership debate Sunday, telling the perceived front-runner that Canadians want more than platitudes.

"I believe that Canadians want to see substance. They don't want empty words,'' Garneau told Trudeau during the debate in Halifax.

"They may not like what I have to say, but at least they will know where I stand.''

Trudeau said he has voiced his views on a variety of issues, citing his opposition to the Northern Gateway oil pipeline and support for the legalization of marijuana as examples.

"I've been just as specific as everybody else,'' Trudeau said during one of the more spirited exchanges in the debate.

Garneau recently challenged Trudeau to a one-on-one debate, an invitation that was declined.

Joyce Murray's proposal to co-operate with the NDP and the Greens to defeat the Conservatives in the 2015 election also drew fire from several candidates.

Trudeau briefly complimented Murray for attracting new supporters to the Liberals, but quickly attacked her idea.

"You have to know that if you make a deal with the NDP, that positive approach is the first thing to go out the window,'' he said.

Garneau asked Murray how she could reconcile her position when NDP Leader Tom Mulcair ``wants to crush us.''

"Marc, as you know, I'm talking about a one-time co-operation,'' Murray replied.

"Kind of like a Canadian hockey team coming together in the Olympics in 2010 and winning gold and then going back and competing with each other once again.''

Martin Cauchon also criticized the proposal and ruled out any merger with the NDP if he wins the Liberal leadership.

A final debate is scheduled for March 23 in Montreal before the Liberals choose their new leader on April 14.

Sunday is also the deadline for the contenders to sign up new supporters for the Liberal party and their candidacies.