McGuinty had only shown up in the legislature twice since he was replaced by Kathleen Wynne as premier in February — both times for confidence votes — but he did not attend Tuesday's vote on the minority government's budget, which passed with support from the New Democrats.
His last public statement was last Friday, hours after the Ontario Provincial Police announced a criminal probe into Conservative complaints that senior Liberals in McGuinty's office deleted emails on cancelled gas plants, saying say he never instructed his staff to destroy government records.
The scandal over the Liberals' decisions to cancel gas plants in Oakville and Mississauga to save seats in the 2011 election haunted McGuinty from the moment the vote reduced his party to a minority government, and never let up as the opposition parties forced public hearings into the energy projects.
McGuinty suddenly prorogued the legislature and announced his resignation as premier last October, hours before the committee hearings were to start into the gas plants.
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He blamed a bitter debate on a rare contempt of parliament motion filed against the Liberals for failing to release documents on the gas plants that ground legislative business to a virtual halt as one of the reasons for quitting.
The former premier testified at the public hearings in May, facing repeated attacks from Progressive Conservative energy critic Vic Fedeli, who compared McGuinty to a criminal who "masterminded the heist, (while) your henchmen committed the crime and drove the getaway car."
McGuinty told the committee it was his decision to cancel the gas plants, saying he took too long to realize the people of Oakville and Mississauga were right to oppose putting the large energy projects so close to homes, schools and hospitals.
McGuinty had repeatedly claimed the cost of cancelling the gas plants would be $230 million, a figure that has since ballooned to $585 million and is expected to grow even higher when the auditor general issues a report in late August.
Also last week, Ontario's privacy commissioner reported that McGuinty's former chief of staff, David Livingston, tried as late as January to find out how to permanently wipe email accounts from government archives, and found Livingston and the former chief of staff to two energy ministers wiped all their gas plant emails, long after the correspondence had been requested by the justice committee.
The Conservatives say they want McGuinty to agree to return to the committee so he can face more questions about the mass destruction of the gas plant emails.
McGuinty, a self-proclaimed education premier, introduced a $1.5 billion a year full-day kindergarten program for four- and five-year olds, but still ended up in a nasty dispute with the province's huge teachers unions over contracts that imposed a two-year wage freeze on many educators.
The former premier served as the MPP for Ottawa-South for 23 years, taking over the seat held by his father, Dalton McGuinty Sr., and held the premier's job for a decade starting in 2003.
McGuinty's resignation would mean Wynne will have to call three byelections. She must call byelections by mid-August in Windsor and London to replace former finance minister Dwight Duncan and former energy minister Chris Bentley, both of whom resigned after Wynne won the Liberal leadership in January.