Now the ticking "time-bomb," in the words of one longtime party insider, has gone off just as Marois was strolling through a trouble-free election campaign.
The possible premier-in-waiting has performed a sudden about-face on the policy and now says that, no, a PQ government would not be forced to hold a vote on independence whenever people gathered a few hundred thousand names on a petition.
Marois now says the petition would simply be taken under advisement. Under the constitutional order, she says, parliamentarians must have the final say on such decisions.
The episode has left constitutional observers, political insiders, and Marois' opponents weighing in on the possible implications on her bid for the premiership.
The PQ has raised, debated and, for a variety of reasons, rejected the idea of allowing citizens to petition for referendums before. Marois herself opposed the plan in 2008.
But amid a wave of discontent within the party last year, an ambitious young member of the PQ caucus took the opportunity to resurrect the moribund idea.
Marois had headed into last summer's legislative recess with her leadership critically weakened by a string of caucus defections. A coup was rumoured to be afoot.
Before dispatching her caucus back to their ridings, she instructed them to return to Quebec City in the fall with some fresh ideas to revive the party's fortunes.
Bernard Drainville, a former Radio-Canada journalist, took Marois at her word and drew up 10 proposals that he said would take the party out of its "bubble," and better connect with citizens.
The proposals included allowing citizen-initiated referendums, of the sort seen in numerous jurisdictions from California to Switzerland. But rather than simply hand his suggestions to Marois, Drainville took the unusual step of making them public.
The move was interpreted as a direct challenge to Marois' leadership.
Considered a likely contender in any future PQ leadership race, Drainville even gave a round of interviews in which he took thinly disguised jabs at his leader.
"If the Parti Quebecois doesn't bring itself closer to the people we won't get through this," he said, referring to the party's low polling numbers at the time.
Drainville tabled the idea again at a PQ convention early this year. This time, Marois was fighting off a potential challenge from former Bloc Quebecois leader Gilles Duceppe.
In a party whose grassroots constantly clamours for measures that might bring Quebec closer to independence, the idea of referendums-on-demand was a surefire crowd-pleaser.
So Marois, grudgingly, went along.
"She has always been against this," political analyst Jean Lapierre told Montreal radio listeners on 98.5 FM on Thursday.
"Gilles Duceppe wanted her job. She was weakened. She was afraid others would leave the party, Bernard Drainville among them. So he rammed it down her throat."
Marois kept her job and citizen-initiated referendums became party policy.
But her unease with the idea became apparent almost immediately.
At a news conference in February to unveil details of the proposal, Marois seemed to offer contradictory interpretations with each answer.
In one instance she said a referendum sparked by a petition would be "binding" on the government. In another, she said "the citizen-initiated referendum is not the referendum in itself," implying a referendum would have to be held on the referendum. A few minutes later she seemed to revert to her original position.
In the meantime, Drainville was giving interviews suggesting the petition would be iron-clad. He was quoted saying that if such a measure had existed in 1990, Quebec would already be independent.
He explained that following the collapse of the Meech Lake accord, with support for sovereignty sky-high, such a petition could have forced then-premier Robert Bourassa, who was a Liberal and not even a sovereigntist, to call a referendum.
Somehow, for the following six months, the issue remained dormant.
It had hardly even come up during the current election campaign, drawing to its close next week. All of that changed when Francois Legault started picking away at the issue.
Marois' rival, the leader of the Coalition party and her one-time cabinet colleague, pressed her on the issue during televised debates this week.
Her replies to Legault prompted journalists to grill her on the issue at a news conference after the debate, on Wednesday night, and again on Thursday.
The barrage of questions so frustrated PQ partisans at one campaign stop Thursday that they started heckling reporters.
A visibly annoyed Marois did clarify her position on the petitions.
If 15 per cent of Quebec's population, or just over 850,000 people, signed a petition the public could essentially ask elected politicians to consider holding a plebiscite, she explained.
"It would force a government to reflect deeply," Marois said.
"Ultimately, it's up to the national assembly to decide when there will be a referendum."
Now analysts, political rivals, and voters are weighing the fallout.
One political scholar expressed bafflement that the PQ would ever have considered the idea.
"Citizen-initiated referendums are not very compatible with a British-inspired parliamentary system," said Antonin-Xavier Fournier, a professor of politics at a junior college in Sherbrooke, Que.
"Citizens themselves can't oblige Parliament and the Crown to adopt a law. It's what we call 'the supremacy of Parliament.'"
This, said Fournier, is hardly top-secret information.
"This is constitutional law 101," he said. "Students of constitutional law in university and CEGEP learn these elementary notions of the supremacy of Parliament."
Drainville acknowledged this point in interviews Thursday.
He told Radio-Canada that the moral weight of 850,000 signatures would bring political pressure on governments to respond, even if they are not legally bound to do so.
What it would also do, he said, is make democracy more relevant to citizens again.
"I believe in this idea that, between elections, we should allow citizens to make themselves heard and to tell a government that isn't listening, 'Hey, listen to us,'" Drainville added in another interview, with 98.5 FM.
Drainville used the example of a corruption inquiry, which Quebecers had been demanding for more than two years before the Charest government finally called one. He said it would have happened faster with the PQ petition plan.
In establishing a 15 per cent threshold, the PQ is setting the bar significantly higher than other jurisdictions that have adopted petition-driven referendums.
In Switzerland, for instance, petitions only require 100,000 signatures — or about 2 per cent of the electorate. That led to the referendum on banning minarets on mosques. The PQ has explained that its rules for the petitions would require that any referendum question would respect Quebec's charter of rights.
That hasn't prevented Marois' political opponents from accusing her of letting PQ hardliners dictate the timing of the next referendum on independence.
Legault calls the hardliners the "caribou," a reference to a determined herd plunging collectively into a ravine. He chastised Marois during this week's debate for allowing her party's rank-and-file to plunge Quebec into uncertainty.
Drainville scoffed at such talk, which he called a scare tactic from Legault.
The PQ has only 90,000 members — barely one-tenth the number needed to reach the petition threshold.
"We're putting the bar very high," Drainville said.
"It would take a heck of a popular movement."
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