The Liberals fell just one seat short of a majority last October, but McGuinty insisted Tuesday his motives were "pure" when he named Witmer to head the Workplace Safety and Insurance Board.
The NDP accused McGuinty of trying to "buy" Witmer's seat to win a majority in the byelection that will be needed to replace her.
"I just don’t believe that’s what motivated Elizabeth Witmer when we asked her," McGuinty said.
When reporters pointed out they were asking his motivation for the surprise appointment, the premier again turned it back on Witmer.
"It takes two, right, it takes two, and when we approached her, I’m sure she gave consideration to all the ramifications," said McGuinty.
"The fact of the matter is she’s the best person available for the job."
Just minutes before suggesting he wasn't motivated by a desire for a majority, McGuinty said Ontario's credit rating was downgraded last week in part because of concerns the province has a minority government.
"The common refrain was: these guys have a minority government and there are risks associated with their capacity to deliver on an ambitious (budget) plan," he said.
The opposition parties said McGuinty nominated Witmer for the job, which pays $188,000 a year, because the Liberals hate being in a minority situation where they aren't fully in control.
"These guys just can’t work with other people so they’re trying to find another route, and that means doing everything they can to buy a seat," said NDP critic Taras Natyshak.
"You’re enticing an Opposition MPP with a lucrative job at a post that she in the past, according to the Liberals, wasn’t fit for."
The New Democrats dug up old quotes from Liberals critical of Witmer's treatment of the WSIB when she was Labour Minister.
The Conservatives insisted they have the utmost respect for Witmer, but say McGuinty was motivated solely by his desire for a majority in offering her the WSIB job.
"No one trusts Dalton McGuinty any more," said PC critic Monte McNaughton.
"He’ll do anything to get a majority government. He’ll spend any amount of taxpayers’ money to do it, so I do question his motives."
McGuinty also dismissed speculation he would call a snap byelection in Kitchener-Waterloo to replace Witmer in hopes of catching the Tories and NDP unprepared.
"I think the people of KW just learned about the vacant seat, so I think we need to let folks kind of adjust to that new reality," he said after touring Blammo Games in a campaign-style event.
"When it comes to these kinds of things my natural inclination is not to move in too quickly, to take our time, so I don’t feel an overwhelming need to get this going."
McGuinty has up to six months to call the byelection.
NDP Leader Andrea Horwath said she doesn't know what will prompt McGuinty to call the vote, but insisted the premier only wanted a byelection to get his majority.
"It’s unfortunate that they were more interested in themselves, in their own power, than they were in the people of Kitchener-Waterloo," said Horwath.
"It wasn’t about Miss Witmer’s capabilities. It was more about what was good for the Liberals in terms of a political opportunity."
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