The astronaut-turned-politician held a news conference to accuse Trudeau of offering only "vague generalities" and empty platitudes during the contest so far.
He challenged his fellow Montreal MP to set out a detailed policy platform, arguing that Trudeau's failure to do so thus far "is the same as asking Canadians to buy a new car without test-driving it."
"He has told Canadians that we need a 'bold plan' and a 'clear vision' without defining either," Garneau said. "On Justin's two clear priorities — the middle class and youth engagement — he has said nothing.
"We have to know what we're voting for, not just who we're voting for."
Garneau's broadside met with a mixed reaction from his caucus colleagues, with some seeing it as a normal sign of a vigorous contest and others seeing it as a sign of desperation in the face of the apparent Trudeau juggernaut.
For his own part, Garneau appeared frustrated that the allegedly insubstantial Trudeau, eldest son of former prime minister Pierre Trudeau, appears to be coasting to victory on little more than his celebrity and his pedigree. Garneau's voice shook with emotion as he appealed to Liberals not to allow another coronation.
"We made the mistake last time of saying, 'All we have to do is choose a leader and everything will work out','' he said, referring to the uncontested crowning of Michael Ignatieff.
"We did not define ourselves, the Conservatives ended up defining us. They'll do it again this time unless we know where each of the candidates stands.
"I am doing the Liberal party a big favour by bringing this up. It's a difficult question but it's one that needs to be asked."
Trudeau has in fact disclosed where he stands on a number of policy fronts, including foreign investment in the oil sands, proposed pipelines from Alberta to the British Columbia coast, the "failed" long gun registry, legalization of marijuana, Senate reform and electoral reform. He recently released a detailed set of proposals for empowering backbenchers and diluting the concentration of power in the Prime Minister's Office.
But Garneau dismissed such policy forays as simply responding to "the topic of the day," as all nine leadership contenders have done.
"What I'm talking about is a very different thing. What I'm talking about is a coherent vision that comes from deep inside as to what the Liberal party stands for and how it will address the future," he said.
In contrast to Trudeau, Garneau boasted that he has unveiled policy planks on "serious, big time" issues such as the knowledge economy, trade, telecommunications, Western Canada, electoral reform, student debt and youth employment.
Trudeau, who was attending a Liberal caucus meeting during Garneau's news conference, chose not to respond immediately.
Later Wednesday, after speaking at Queen's University in Kingston, Ont., the front-runner made no apologies for his refusal to unveil a detailed platform. While he's taken strong positions on a wide range of issues, Trudeau said his campaign is aimed at involving Canadians in politics again, including policy development.
"What I bring to the table is the capacity to draw people in, to believe and participate in politics once again," he said.
"I've talked about a lot of substance and I'll continue to ... We just have to be sure that we're leaving room for Canadians in the development of solutions that are going to carry us into a better place as of 2015."
Trudeau has repeatedly maintained that the leadership contest is not the time to unveil a detailed platform for the next election in 2015, arguing that platform development should not be a top-down exercise that reflects only the views of the leader's inner circle.
Garneau insisted he's being "constructive" and shrugged off suggestions his attack on Trudeau risks giving the Conservatives ammunition for their next set of attack ads. Embarrassing clips from the 2006 Liberal leadership contest were used to great effect in Tory ads that ridiculed the last two Grit leaders, Stephane Dion and Ignatieff.
Nova Scotia MP Scott Brison, who ran for the party leadership in 2006 and has not publicly endorsed anyone in the current contest, said Garneau's criticism of Trudeau is "not justified or accurate or helpful."
"I don't think we need to give any aid to Conservative negative attack ads," he said.
Brison added that he's "disappointed" in Garneau, saying his attack is "not in character" and smacks of desperation.
"I've been there. I've been a candidate when things weren't going so well in a leadership race, so I can understand the motivation to shake things up. But you have to be very careful ... that you don't jettison somebody else who will ultimately not just be leader but is very well positioned to become prime minister."
According to the most recent financial reports filed with Elections Canada, Trudeau is miles ahead of the eight other contenders in terms of fundraising, while Garneau is a distant third. Trudeau had raised almost $700,000 by the end of last year, while Martha Hall Findlay had raised about $150,000 and Garneau just over $122,000.
Trudeau has been endorsed by 19 fellow MPs so far, while Garneau has captured the support of three. No other contender has yet snagged a caucus endorsement.
Interim Liberal leader Bob Rae called Garneau's broadside a normal part of the leadership campaign process.
"Politics is a lot more like hockey than it is like ballet. So there's going to be some vigorous contact," Rae said.
The race ends April 14.
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