"I think Kathleen needs a free hand, and there are a lot of talented people in our caucus who have not had the opportunity to serve in cabinet, (and) can help put a fresh face on the government," Duncan said at the Ontario legislature.
Duncan said he plans to quit as the MPP for Windsor-Tecumseh, effective Feb. 14, which will force Wynne to call a byelection within six months.
Meanwhile, Northern Development Minister Rick Bartolucci announced he would not seek re-election in Sudbury, but would not resign his seat. Bartolucci also said he would not be in Wynne's new cabinet when it is sworn-in next Monday.
Duncan, who oversaw the introduction of the harmonized sales tax in Ontario and the bail out of the auto companies during the recession, admitted he won't miss being finance minister, especially when spending is severely limited by the $11.9 billion deficit.
"It's a very lonely job in the sense that you're the one who has to bring all the bad news to your colleagues in a good economy or a bad economy, and you're very isolated," he said.
"You're very much alone within your cabinet and caucus because you're always Dr. No, and you can't flinch."
The Progressive Conservatives said Duncan's budgets added way too much to Ontario's net debt, which was at $237.6 billion as of March 31, 2012.
"I'll give him some credit for saying that the province's debt is a ticking time bomb, I think it's an important recognition," said Opposition Leader Tim Hudak.
"The irony, of course, is he's the one that lit the fuse."
Duncan's lengthy career in public life — he'd been a political aide at the legislature before being first elected in 1995 — was "nothing to sneeze at," said NDP Leader Andrea Horwath.
"He's served in a number of different capacities and that's something we should recognize for what it is," said Horwath. "It's not an easy row to hoe."
Duncan, who also served as Ontario's energy minister, insisted he hasn't yet accepted any of the job offers he's received, and left the door open to running federally at some point, but added he "may never walk through it."
The Liberals weren't the only ones to add to Ontario's debt in recent years, said Duncan.
"Four governments of three political stripes since 1990 have each in their succession doubled the province's debt," he said.
"Only one year in that generation has the province's debt been actually reduced, and that was due to an accounting change."
Duncan had telescoped his resignation for months by saying he was prepared to quit his Windsor-Tecumseh seat if former Windsor-West MPP Sandra Pupatello had won the Liberal leadership. Right after Wynne won the top job Jan. 26, Duncan said he would not be in her new cabinet and would likely resign his seat within weeks.
Duncan lost when he ran against McGuinty for the Liberal leadership in 1996, but became his finance minister after Greg Sorbara bowed out.
"Dwight Duncan was the longest serving finance minister in the modern era and Ontario is better for it — stronger, fairer and more prepared for a future of constant change and ever-growing opportunity," McGuinty said in a statement.
McGuinty also praised Duncan for reforming Ontario's tax system and for working to protect the auto industry during the recession.
"Dwight's steady hand has set our province on a sure path to a balanced budget while protecting the gains we have made together in health care and education."
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