The Ontario legislature may have adjourned for the summer holidays, but the Progressive Conservatives aren't taking any breaks from hammering the Liberals, rolling out a new attack ad this week.

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A new attack ad from the Progressive Conservatives links Ontario Liberal Premier Kathleen Wynne, left, with former premier Dalton McGuinty on the handling of the cancelled gas plants affair. (Chris Young/Canadian Press)

The commercial takes aim at Premier Kathleen Wynne and attempts to link her with her predecessor, Dalton McGuinty, over the cancellations of two Toronto-area power plants in order to save five Liberal seats in 2011. The cancellation of the plans is expected to cost taxpayers $585 million.

The 30-second TV spot  titled Kathleen Wynne: Nothing has changed is being released just before a former McGuinty chief of staff is set to testify before a committee probing the power plants scandal.

"Kathleen Wynne and Dalton McGuinty. Nothing has changed," a voice-over says in the ad.

The commercial claims that Wynne "signed a key document authorizing the cancellation, claiming the cost of cancelling was $40 million but knew it would be higher — almost $600 million dollars, and counting."

'Same old Liberal policies'

It accuses the current premier and McGuinty of colluding to low-ball the cancellation costs by originally stating the price tag would only be around $230 million.

PC Leader Tim Hudak said the intention is to make it "crystal clear" for voters that a Liberal government under Wynne's leadership will be an era of "more of the same."

'I understand that the Opposition is going to do what the Opposition is going to do.'—Ontario Liberal Premier Kathleen Wynne

"I mean the face may be different, maybe it's a different voice, but it's the exact same old Liberal policies," Hudak said.

In Peterborough, Ont.,  on Monday morning, Wynne shrugged off the latest criticisms as a typical, if not predictable, political manoeuvre.

"I understand that the Opposition is going to do what the Opposition is going to do," she said. "I need to stay focused on the growth of the economy in Ontario."

The ad comes ahead of a series of three byelections in the ridings where McGuinty and two of his former cabinet ministers, Dwight Duncan and Chris Bentley, had served until recently.

With the departure of the former premier and the two cabinet ministers, the governing Liberals hold just 50 of the 107 seats in the Ontario legislature. The Progressive Conservatives have 36 seats and the New Democrats another 18.

Wynne has not yet scheduled the forthcoming byelections and they do not have to be called until later this summer.

Chris Morley, a former chief of staff for McGuinty, is scheduled to testify on Tuesday before a legislative committee hearing on the power plants fiasco.

Morley could be asked to explain why senior Liberal staff in McGuinty's office deleted emails on the gas plants and tried to permanently wipe their email accounts from government databases.

The controversy over the pair of cancelled gas plants has raged for months, even before McGuinty announced last fall that he was going to be stepping down as leader of the Ontario Liberals.

In addition to the investigation by the legislative committee, Ontario’s auditor general, Jim McCarter, has been probing the costs associated with the cancelled plants — he has already reported on the estimated cost of scrapping a gas plant in Mississauga, Ont. His office will address the Oakville plant in a separate report.

With files from the CBC's Mike Crawley