The Progressive Conservatives are making the Liberals' decision to cancel plans for two gas-fired power plants the focus of their campaign to win a by-election in Ottawa South and claim a riding held by former premier Dalton McGuinty for two decades.

The cost of cancelling plants in Oakville and Mississauga cost taxpayers an estimated $585 million, well above the figures initially quoted by the governing Liberals.

The total costs have been updated in recent months just in time for the by-elections, which was perfect timing for the Conservatives, according to Luc Turgeon, a University of Ottawa professor with the school of political studies.

The gas plant accusations are also easy fodder for Ottawa South PC candidate Matt Young, a 34-year-old business manager, said Turgeon.

"He's mostly running a campaign that is attacking Mr. Fraser," said Turgeon. "In the context of a by-election, (Young) doesn't have to answer questions about his party's platform."

PC leader Hudak in Ottawa

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PC candidate Matt Young was on the campaing trail Sunday and he will be joined by his party leader Monday for canvassing. (CBC)

On Monday, PC leader Tim Hudak visits Ottawa with just three days remaining until the by-election, and just one day after Ontario Premier Kathleen Wynne visited the Ottawa Farmer's Market at Brewer Park with Liberal candidate, John Fraser.

Signs are pointing to Ottawa South as one of the tightest races of Thursday's five provincial by-elections.

The riding has been a Liberal — or rather, a McGuinty — stronghold for the last 26 years, as McGuinty's father held the seat before the former premier.

The PCs held the seat for the previous six decades.

Liberals rely on door-to-door momentum

At the beginning of the campaign, Young called the fight against the Liberals "David vs. Goliath," though poll numbers suggested it was a close race. Polls leading up to the by-election now have Young leading.

"We didn't know we'd have such a wind at our back and that so many people would be out supporting us," Young said last week.

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Ontario Premier Kathleen Wynne joined Liberal candidate John Fraser on Sunday at the Ottawa Farmer's Market. (CBC)

Fraser said polls matter less than what people do on election day.

"The most important poll is the one that's on election day. I have a singular focus, which is meet as many people as I can in this 28-day period," he said Sunday.

NDP candidate Bronwyn Funiciello is also campaigning door-to-door Monday with the support of CUPE Ontario president Fred Hahn.