There is a lot of historical precedent for prorogation, said Duncan.
"I completely support the premier's decision on that," he said.
"I think prorogation is a very ancient tradition of Parliament...particularly in minority houses."
McGuinty suddenly decided Oct. 15 to resign as Liberal leader and prorogue the legislature, and his deputy premier was forced on the defensive Tuesday when he met the media for an unrelated announcement.
"I think it was the appropriate decision in the circumstances and I support doing it," added Duncan.
The Progressive Conservatives and New Democrats are fuming over prorogation and demanding McGuinty recall the legislature long before the Liberals pick a new leader in late January. McGuinty said last week he wants his successor to decide when the legislature will be recalled.
The Tories lashed out at Duncan, a potential leadership candidate, for supporting McGuinty's move to shut down all legislative business, including committees.
"Prorogation is a very, very serious thing and at a time when Ontario is literally on fire from a fiscal perspective, he's got no business making an irresponsible statement like that," said PC critic Peter Shurman.
The New Democrats said people are outraged by McGuinty's move to prorogue the legislature until at least next February, which they said demeans all politicians.
"People expect us to be here in the legislature addressing the big questions, so when people see that in fact the legislature is shut down, it makes us look non-credible," said NDP critic Peter Tabuns.
"When they see this, it disgusts them frankly."
The opposition parties say the only reason McGuinty prorogued was to shut down scheduled committee hearings into the Liberals' decision to cancel generating stations in Oakville and Mississauga, at a cost to taxpayers of at least $230 million.
The Tories and NDP estimate the actual cost of cancelling the gas plants is closer to $1 billion, and say the money was spent to save Liberal seats in last fall's election.
The New Democrats said people are entitled to know what the final cost of the gas plant decisions will be, and why the government tried to hide relevant documents.
"This government is using a procedural tool to keep Ontarians in the dark about the truth of the gas plant cancellations," said Tabuns.
All Conservative MPPs signed a letter to McGuinty on Tuesday asking that he immediately recall the legislature to deal with the scandal involving a police probe of the Ornge air ambulance service and the contempt motion over the government's initial refusal to release all the documents on the cancelled gas plants.
"Your decision to shut down the legislature means a government out-of-control has been put on auto pilot," wrote the Tories.
"Your scandals remain unaccounted for, your reckless overspending remains unchecked and Ontario's debt continues to accumulate at record levels."
The opposition can revisit the gas plant issue when the legislature resumes sitting sometime next spring, said Duncan.
However, most observers believe it's doubtful there will be much of a spring session before Ontario is plunged into another general election.
The new Liberal leader is expected to recall the legislature in late February or early March with a Throne Speech, quickly followed by a budget and then another prorogration to allow for an election.
Duncan admitted he's still considering whether or not to run for leader to replace McGuinty.
"You're looking at probably a five to ten year commitment of your life, and that's the one that's front and centre for me," he said.
"It's a big call, and I'll have it pretty soon."
Several other potential leadership candidates are said to be waiting to see if Duncan enters the race before making their decisions.
McGuinty told any Liberal ministers that want his job they'll have to resign their cabinet posts by Nov. 23 in order to enter the leadership race.
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