The Liberal leadership race has been a generally genteel affair so far, but Marc Garneau has changed tacks, taking the fight to front-runner Justin Trudeau.

Garneau has accused Trudeau of failing to detail what he stands for and where he would take the party if he wins the top job.

"As Liberals, we cannot wait until after the leadership race is over to find out what we signed up for," Garneau said in a news release Wednesday.

"That is like asking Canadians to buy a new car without first test-driving it."

The astronaut-turned-politician says he's laid out his plans and wants Trudeau to do the same.

Garneau said he has outlined his views on the knowledge economy, trade, telecommunications, Western Canada, electoral reform, and student debt and youth employment.

"This is not the case with Justin," he said. "He has told Canadians that we need a 'bold' plan and a 'clear vision' without defining either.

"On Justin's two clear priorities, the middle class and youth engagement, he has said nothing. Therein lies the fundamental difference between Justin Trudeau and myself."

The party has had coronations in the past and they turned out to be mistakes, he said.

Garneau said he's not concerned he's providing ammunition for the next set of Conservative attack ads, which eviscerated the last two Liberal leaders.

He insisted he's being "constructive."

Garneau and Trudeau are among nine candidates vying for the leadership. The race ends April 14.

Until now, the leadership candidates have taken only veiled shots at Trudeau in a race with little rancour.

Photo: Liberal leadership candidate Marc Garneau speaks during a press conference on Parliament Hill in Ottawa on Feb. 13, 2013. THE CANADIAN PRESS/Sean Kilpatrick"