Liberal leadership hopeful Marc Garneau has added inexperience and immaturity to the list of front-runner Justin Trudeau's alleged shortcomings.

Garneau, who last week accused the prohibitive favourite of offering little but empty platitudes, now says Trudeau has needlessly inflamed Quebec sovereigntists.

He says Trudeau made "a rookie mistake" when he appeared to suggest at least 66 per cent of Quebecers would need to vote for independence before the federal government would agree to negotiate secession.

Trudeau made the remark — which he later clarified — in response to a question from a student at McGill University on Wednesday.

Garneau says Trudeau should simply have reiterated his support for the Clarity Act — which stipulates that an undefined "clear majority" would be required — and moved on immediately to economic issues that are of far more concern to Quebecers.

That, he says, would have been the "more mature" approach.

"I think it was a rookie mistake by Justin, getting sucked into it," Garneau said in an interview Thursday.

"What he did was to give the sovereigntists an opportunity to sort of get indignant," he added

"I think a more mature, if I can put it that way, approach would be to focus on the issues that really are a preoccupation to Quebecers ... and not allow himself to be dragged into it."

Two-thirds better threshold than 50+1?

Trudeau waded into the issue after being asked about the NDP's proposal to replace the Clarity Act with a bill that stipulates that a bare majority of 50 per cent plus one vote would be sufficient to trigger secession negotiations.

"We should at least have the same threshold needed to change the New Democratic Party's constitution, which is two-thirds," he said.

Later, Trudeau clarified that he supports the Clarity Act, which he said was based on the Supreme Court's advice that the threshold "is a political decision that should be taken at the time and place," after a referendum vote, taking into account voter turnout and other factors.

Quebec Premier Pauline Marois termed Trudeau's two-thirds remark a provocation. And her intergovernmental affairs minister, Alexandre Cloutier, called it "irresponsible," "anti-democratic," contrary to Quebec law and history and disrespectful of Quebecers.