MONTREAL--Quebec's election landscape shifted over the weekend with the confirmation a former Montreal police chief and anti-corruption whistleblower will join a new political party.

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