Wynne called the byelections in Windsor, London, Ottawa and two Toronto ridings to replace five Liberals who have resigned since she became premier in February.
NDP Leader Andrea Horwath said the Liberals were hoping that people in those ridings were going to be busy getting ready for a long summer weekend instead of thinking about politics.
"It's obviously very cynical, and it is another one of those ploys the Liberals use," she said.
The Progressive Conservatives went even further, accusing Wynne of trying to "subvert democracy" by holding the byelections in the dead of summer.
"Democracy is a fragile institution and it relies on the behaviour of political parties to not only follow the rules to the full letter of the law, but also the spirit of the law," said PC critic Lisa MacLeod.
"(Wynne) has decided she doesn't want to do that. I feel it's akin to actually stuffing the ballot boxes."
The non-profit, non-partisan citizens' group Democracy Watch also took Wynne to task for calling the votes just before a summer long weekend, and for not giving the other parties enough time to nominate candidates.
"Wynne's decision ... is as dangerously undemocratic and self-interested as former premier Dalton McGuinty's decision to shut down the legislature last fall so the Liberals could hold their party leadership race," Democracy Watch posted on its website.
"The decision is designed to suppress voter turnout, especially by young voters who often work away from home during the summer."
Transportation Minister Glen Murray — subbing for the vacationing Wynne — predicted turnout would be higher than normal because the large number of byelections will generate more publicity and voter interest.
"In some sense it's a bit of a mini-election, so people become aware of it and it's a lot easier to communicate," he said.
It's "ridiculous" for the Tories to complain about summer byelections, added Murray.
"I'm not sure how they'd be upset by five byelections when only weeks ago they were trying to force a general election that would have had to occur in the middle of summer as well," he said.
The NDP is expected to take Windsor-Tecumseh from the Liberals, while the governing party is believed to hold the edge in McGuinty's old Ottawa-South seat and in the two Toronto ridings, Scarborough-Guildwood and Etobicoke-Lakeshore.
London-West is considered a close two-way race between the Liberals and Tories, but the NDP say they are also in contention in that riding.
However, despite all the controversy over the $585 million spent to cancel gas plants in Oakville and Mississauga, which the Conservatives say shows the Liberals are a corrupt government, the Tories were downplaying their chances in the five votes.
"There is an uphill battle because the machinery is already in place for the Liberals," said MacLeod.
"We know that Kathleen Wynne will spend taxpayers' dollars to steal election seats — she did that in the 2011 election campaign — but are we giving up? Hell no."
The opposition parties want voters to use the byelections to punish the government for the decisions to cancel the gas plants, which they say was done to save Liberal seats in the 2011 election, but agree local issues could play an even bigger role.
"People are concerned about the gas plants, yes, but they are also concerned about their health-care system, they're concerned about the lack of real jobs for folks who lost jobs during the recession," said Horwath.
The Liberals were one seat short of a majority government before the five resignations, so the outcomes of the byelections will not change the balance of power in the legislature.
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