The Montreal MP threw down the gauntlet Monday, arguing that Trudeau has undermined his own credibility and that of the Liberal party by speaking only in "vague generalities" thus far.
"It is time for Justin to step up and defend his ideas because if he can't defend them to me in front of a room full of Liberals, he will never be able to do it against (Prime Minister) Stephen Harper," Garneau told a news conference.
"The leadership of the Liberal party is too important a position to be handed to an untested candidate who is hiding behind a carefully crafted public relations campaign."
Trudeau quickly rebuffed the challenge.
"I respect all the candidates" running for the Liberal leadership, he said via Twitter.
"See you in Halifax, Marc. I hear there are 1 on 1s."
He was referring to the fourth of five all-candidates' debates, which is scheduled to take place Sunday in Halifax. It will include a series of one-on-one encounters among the contenders, as did the last debate in Mississauga.
However, Garneau said the debates have provided "only fleeting moments for substantive debate," given the number of candidates. He calculated he's had only three minutes so far of direct face time with Trudeau.
The crowded field thinned out somewhat later Monday as Toronto technology lawyer George Takach dropped out and threw his support to Trudeau.
His departure still leaves eight candidates in the contest, however — an unwieldy number to accommodate in the two hours allotted for each of the remaining two debates sponsored by the party.
Garneau is proposing an unofficial, one-hour debate, moderated by a journalist, in which he and Trudeau can square off, much as Barack Obama and Mitt Romney did during last fall's presidential race in the United States.
Asked why Trudeau should debate him alone, excluding the other candidates, Garneau essentially said it's a two-man race.
"I believe that most of the polls show me as being definitely one of the two and the second and I think I'm catching up.
"And I really want Canadians to have the option of seeing very, very clearly, in much more detail, where the two front-runners are standing."
Garneau added that other candidates are free to challenge Trudeau to a one-on-one debate of their own.
Whether Garneau is actually the second-place contender is debatable.
Opinion polls have suggested that Trudeau is by far the favourite, among Canadians, with Garneau, the country's first astronaut, a distant second. Those polls are not considered particularly meaningful, reflecting name recognition as much as anything.
There have been no polls of Liberal party members and supporters — the people who are actually eligible to vote. The outcome will depend largely on the ability of candidates to recruit supporters and then persuade them to cast ballots.
Fundraising is the only measurable indicator so far of how each camp is faring. As of the end of 2012, financial reports filed with Elections Canada show Trudeau had raised almost $700,000. Martha Hall Findlay was a distant second with almost $150,000 while Garneau was third with $123,000.
However, Garneau's camp said Hall Findlay's numbers actually included about $35,000 in money raised to pay off debts from her 2006 leadership run. Comparing only donations raised for the current contest, Garneau had raised slightly more than Hall Findlay by the end of last year.
In a statement, Hall Findlay said her camp has suggested that a smaller debate with "the three or four front-runners" would be more illuminating for Liberals, some of whom have expressed frustration with the various debate formats tried by the party thus far.
However, she noted that all contenders have made "a significant personal and financial commitment to this leadership race" and have been putting forward "some interesting ideas."
"All should be heard," she concluded.
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