Tom Mulcair found himself on the defensive Monday over his party's policy on Quebec secession as he kick-started an eight-day pre-election tour of Ontario.

The NDP leader defended his position that a bare majority of 50 per cent plus one vote should be sufficient to trigger negotiations on Quebec's separation from Canada.

And he lashed out at Liberals, who've been the most critical of the NDP approach, accusing them of "giving up" on the majority of Quebecers and trying to re-stoke old quarrels.

"I haven't given up on the majority of Quebecers, unlike the Liberals who have," Mulcair said at the launch of his tour in downtown Toronto, what he called "Canada's most important city."

In turn, Liberal Leader Justin Trudeau, doing his own pre-election campaigning in New Brunswick, accused Mulcair of pandering to Quebec nationalists at the expense of the country's unity.

"The job of a prime minister is to bring the country together and protect our unity, it's not to make it easier for the country to be broken up," Trudeau said in a phone interview from Miramichi.

Liberals 'not serious'

The NDP has vowed to repeal the Clarity Act, introduced by the Chretien Liberal government in the wake of the razor-thin No victory in the 1995 referendum on Quebec independence.

The Clarity Act states that a clear majority vote on a clear question on secession would be required before the federal government would agree to negotiate a divorce.

The NDP is promising to replace the act with new legislation that would set the threshold for triggering separation talks at 50 per cent plus one, provided the referendum question was clear and there were no irregularities in the vote.

Mulcair criticized the Clarity Act on Monday for failing to specify the precise threshold needed.

"(Liberals) think that they need a stratagem, some sort of game-playing politically to say, 'Well, we won't tell you what the number is but it's whatever they get plus a whole bunch more.' Well, that's not serious," he said in response to questions from the media.

Mulcair noted that the "mother of all Parliaments," in the United Kingdom, accepted a simple majority as the threshold that had to be met in the recent referendum on Scottish independence.

"People have to understand that yes means yes. Yes can't mean, 'Oh, perhaps we want a better deal'," he said.

Trudeau countered that the Clarity Act is based on a landmark ruling of the Supreme Court of Canada in 2000; "it's not about anyone playing games."

Mulcair's stand 'very concerning': Trudeau

The top court said both the referendum question and the result must be "free of ambiguity" before the federal government would be obligated to negotiate separation. It did not specify a threshold but repeatedly indicated something more than a bare majority would be necessary, at one point noting that "Canadians have never accepted that ours is a system of simple majority rule."

"Mr. Mulcair is disagreeing with the Supreme Court on something as basic as the unity of the country," said Trudeau, calling that "astonishing and very concerning."

"This one is even bigger than any fight that Mr. (Prime Minister Stephen) Harper has picked with the Supreme Court. This is about what it would take for Canada to continue to exist or to be broken up."

Mulcair, who noted he's the only federal leader to have actively campaigned against secession in two referendums, said the NDP is the first federalist party in a generation to win a majority in Quebec. And he credited the party's "open, optimistic, positive approach" toward involving Quebecers in the country's affairs, rather than the Liberal approach of "constantly provoking battle with Quebecers."

"I take a backseat to no one when it comes to defending this extraordinary country of ours," Mulcair said.

Mulcair's defence of the NDP's policy on Quebec secession came as he stood on the sweltering roof-top of a Toronto housing co-operative, flanked by NDP MPs and candidates for the Toronto area.

The event marked the start of an eight-day tour of Ontario, which accounts for more than one third of the 338 seats that will be up for grabs in the Oct. 19 federal election.