MONTREAL - The Quebec Liberals' slim four-seat majority will come under pressure this fall with an Opposition-led effort to weaken party unity during a corruption scandal.

Premier Jean Charest's opponents have made it clear they will seek to sow divisions in his famously tight-knit caucus as soon as the legislature resumes sitting on Tuesday.

Parti Quebecois Leader Pauline Marois announced she would immediately table a motion the following day calling for a public inquiry into corruption in the construction industry.

In essence, she encouraged a Liberal mutiny.

In a public message to Liberal backbenchers, Marois warned that they have two options: either protect the interests of Quebec voters, or those of the Mafia and the Liberal party.

Marois is challenging Charest to hold a free vote on her motion while also appealing to Liberal backbenchers to join the popular movement demanding an inquiry.

"I am asking Mr. Charest ... to act like a democrat, to liberate the Liberal members from the code of silence that he imposes on them," she told a Quebec City news conference.

"I am imploring the Liberal members: You have to act with full freedom."

Charest's Liberals have up to two years left in their mandate because they were re-elected in late 2008; but that term could be cut short if Charest's party, which has seen several senior members quit recently, loses more members and sees its majority disappear.

The premier appeared to have weathered the worst of the corruption scandals that have been raining down on his province over the last two years.

But his task was complicated last week when an explosive internal report by a government anti-collusion task force was leaked to the media.

It describes a "clandestine universe" well beyond the scale previously imagined, rife with corruption and ties between organized crime, the construction industry and political parties.

Charest has spent the last two years dismissing the need for a public inquiry, arguing that the matter was best left to police.

Last week's revelations were not enough to sway the premier. He brushed aside the idea of holding an inquiry even before having read the task force's report.

Despite his current woes, Charest won't be easily dislodged. His party has maintained near-total unity during eight turbulent years in power.

Chief Government Whip Lucie Charlebois said she doesn't expect that to change following the release of the corruption report.

She dismissed Marois's description of a caucus held in check only by Charest's iron fist. She said the premier allows disagreements behind closed doors, which she said makes it easier for members to toe the party line in public.

"We have our discussions in caucus, we have our exchanges in caucus, everybody has a chance to express their opinion," Charlebois said in a recent interview.

"When we finish our discussion it's like a family. A couple can't agree on everything but at a certain point you have to come to a consensus."

Currently the Liberals have 64 seats in the 125-seat legislature, though that includes speaker Jacques Chagnon who only votes in case of a tie. With one seat vacant, the most the opposition can must muster is 60 votes.

There's another, perhaps bigger, challenge for Marois: maintaining unity on her own benches.

Her troubles were underscored Monday when she called a news conference to attack Charest's troops — then cut it short after she faced two awkward questions about the PQ.

The province's sovereigntist movement continued to splinter Monday as a former Parti Quebecois member announced the creation of yet another political party.

With that gesture, the PQ's summer of discontent dragged on into fall. The PQ has been mired in existential debates since five high-profile members quit, ostensibly over Marois' handling of the Quebec City arena file.

Their move appeared to trigger a series of manifestos from sovereigntist fellow travellers calling for an overhaul of the independence project, and with it Quebec's entire political culture.

By the end of summer, around 400 disgruntled sovereigntists had gathered to form their own group, while criticizing the PQ and its leader. That group stopped short of turning itself into a distinct political party.

On the eve of the legislature's return, however, former PQ finance critic Jean-Martin Aussant announced he was forming his own political party, saying sovereigntist voters deserved a stronger vehicle.

"I came to the conclusion that another party was needed to send a clear message to Quebecers that we have to take our future in our hands, that we must become masters of our own destiny again," Aussant, a onetime rising star in the PQ, told a Quebec City news conference.

Aussant's party has been provisionally dubbed "Option nationale."