The third-most popular party in Quebec is hoping a star candidate who has just been at odds with the government will boost its hopes of vaulting into first place in the polls.

Jacques Duchesneau, who led the charge for an inquiry into corruption in the province's construction industry, announced Sunday he will run for the upstart Coalition for Quebec's Future in the Sept. 4 election.

At a news conference, Duchesneau said he was also approached to run for the Parti Quebecois as well, but decided the Coalition was best suited to "clean up" the province.

"I think it's the only party capable of bringing change, the urgent change required for Quebecers," he said in his riding of St-Jerome, northwest of Montreal.

The announcement, which had been anticipated for several days, gives the third-place party an immediate boost in credibility as it tries to make the fight against Quebec's corruption troubles a central issue for voters.

"It brings some extra attention to the (Coalition for Quebec's Future) at a time the party obviously needs it," said Bruce Hicks, a political science professor at Concordia University.

"It should shore up their support."

Duchesneau was hired by Premier Jean Charest's Liberals in 2010 to investigate allegations of corruption and collusion in the construction industry. His bombshell report, leaked to the media last fall, drove Charest to call a public inquiry into the construction industry.

Duchesneau revealed at the inquiry in June that he had leaked the document, saying he gave it to a journalist because he was convinced the government was planning to ignore it.

Coalition leader Francois Legault, who was beaming at the announcement on Sunday, said Duchesneau would serve as deputy premier if his party takes power.

"No other party can count on such a powerful force," Legault said.

"His expertise in criminal investigations, in fighting organized crime, will allow him to quickly identify flaws in the system and find lasting solutions so that we can finally rid ourselves of the cancer of corruption in Quebec."

After shrugging off reports of Duchesneau's candidacy a day earlier, Charest came out swinging on Sunday.

He called the move political opportunism and accused Duchesneau of demagoguery for making unfounded accusations against his party.

Charest said his government had already taken steps to tackle corruption and illegal political financing, including calling an inquiry. He listed eight new pieces of legislation introduced by his government aimed at curbing the problem.

"I think these actions speak for themselves," he said at a news conference in Levis.

Charest gave his government an eight out of ten in dealing with the problem on Thursday. Duchesneau said hearing that assessment cemented his decision to run for the Coalition, telling reporters he would have given Charest a two out of ten.

Meanwhile, PQ Leader Pauline Marois avoided commenting on Duchesneau.

She took no questions on the subject from reporters, offering up Bernard Drainville, the PQ's critic for intergovernmental affairs, instead. Drainville said Duchesneau isn't the only one committed to asking questions and pushing an inquiry.

Quebec Solidaire, a party that held one seat in the provincial legislature, also tried to lay claim to the ethics issue. Spokeswoman Francoise David said her party had, for several years, been a "true champion in the fight against corruption."

Still, there's no doubt the move shifted the spotlight onto the Coalition for Quebec's Future during the first weekend of the campaign. It clearly overshadowed a big PQ campaign rally, featuring Marois and several candidates, along with an announcement from Charest to increase the province's $7-a-day daycare program to just under $8 by 2017.

In the longer term, Duchesneau's impact on the election campaign is more difficult to predict. The 63-year-old has "a history of a lack of filter," Hicks said, including dust-ups with several of the province's top political figures.

Earlier this year, for instance, he told a newspaper he wouldn't trust Montreal Mayor Gerald Tremblay to sell him a used bicycle. Tremblay demanded an apology.

He has also produced a second report on corruption, which he prepared on his own, that has never surfaced publicly. He tabled it with the inquiry in June.

At the inquiry, Duchesneau said that Quebec political parties are funded by "dirty money," with up to 70 per cent of cash raised illegally.

"He's very much a police chief in character," Hicks said. "He throws out allegations, but he doesn't always prove them beyond a reasonable doubt."

Duchesneau said Sunday he wouldn't make the report public, leaving it up to the inquiry to use as evidence. He said his job now is to be a politician.

- with files from Martin Ouellet

Related on HuffPost:

Loading Slideshow

• Key Quebec Election Issues

  As Quebec begins a provincial election campaign, with a vote scheduled for Sept. 4, here are some key issues and the stated positions, so far, of the three largest parties: the Liberals, the Parti Quebecois and the Coalition for Quebec's Future.<br><br><em>With files from CP</em>

• Student Strikes

  Liberals say their $254-a-year, seven-year tuition increases will improve universities while expanded loans and bursaries programs will actually leave the poorest students better off. Liberals have mostly refused to budge in face of protests, although their original proposal was for $325-a-year increases over five years. Their controversial Bill 78 would reopen classes in mid-August for one-third of students still on strike, while setting out severe fines for anyone blocking schools.<br><br>PQ has been more supportive of protesters and would cancel the hikes, propose smaller increases pegged to inflation and hold provincial summit on university funding.<br><br>The Coalition has positioned itself to occupy the middle ground, proposing more modest annual tuition increases of $200 a year over five years. Party originally voted for Bill 78 but now says it created unnecessary tension and wants some provisions suspended.

• Ethics

  After two years of intense pressure, Charest Liberals called a corruption inquiry that is now probing malfeasance in construction industry and its ties to political parties and organized crime. Before that, they had announced plans to hire more oversight officials at Transport Department; tougher fines for engineering firms; stricter political fundraising laws; new rules for public-works tendering; and new anti-corruption squad that has since made numerous arrests.<br><br>PQ making ethics central plank of platform. It wants tougher legislation preventing companies guilty of tax evasion from winning public contracts. It also proposes new measures to combat voter cynicism including: citizen-initiated referendums, fixed election dates, political donations limited to $100 a year, and the right to vote at age 16.<br><br>The Coalition wants new integrity commissioner to oversee government contracts, and new powers for prosecutors, as part of a "big cleanup." It also promises fixed election dates.

• Northern Development

  Liberals will tout Plan Nord, a sweeping plan that sets out $80 billion in public and private investments in mining, energy, infrastructure and conservation projects over a quarter-century.<br><br>PQ accuses Liberals of selling off Quebec's natural wealth at cut-rate prices and is calling for a 30 per cent surtax on profits from non-renewable resources.<br><br>The Coalition has also taken aim at the signature plan, alleging windfall will primarily benefit foreign companies and Quebec mining firms cosy with Liberals.

• Sovereignty

  Liberals have long stood as the major federalist option in Quebec. Party is frequently accused by opponents of being subservient to Ottawa. However, it has clashed publicly with federal government over issues like long-gun registry, omnibus crime bill and changes to health transfers.<br><br>PQ is offering no timetable for third referendum on independence. Instead, party plans to pick fights with Ottawa in seeking more power over immigration, environment, agriculture and revenue collection. PQ hopes such battles will generate support for independence. Eventually, Quebecers themselves could initiate referendum, under plan to allow California-style plebiscites. People would need to collect 850,000 signatures to hold provincial vote on a given topic.<br><br>The Coalition, led by former PQ minister Francois Legault, vows to shelve any referendum on independence for 10 years to focus on building economy. But many federalists remain wary of the once-passionate sovereigntist.