The votes are being held in Windsor, London, Ottawa and two Toronto ridings to replace five Liberals who quit this year, including former premier Dalton McGuinty.
The byelections are a mini-referendum on Kathleen Wynne's six months as premier, a chance to see if voters will hold her responsible for scandals such as the $585 million spent to cancel two gas plants, which happened on McGuinty's watch.
"Byelections are challenging for governments. There is no question about that," Wynne said as she, like all the party leaders, downplayed expectations in advance of the votes.
The Progressive Conservatives hope people use the byelections to register protest votes against the Liberals for cancelling two gas plants to save seats in the 2011 election, for doubling the deficit and for financial problems and a police probe at the Ornge air ambulance service.
"I think it’s a chance for folks to look at the scandals, to look at the mess and say 'You know what, enough is enough. We want change,'" said PC Leader Tim Hudak.
The New Democrats propped up the minority government this year, helping pass the budget to avoid a general election, and is not worried voters will give the Liberals credit for NDP ideas such as cutting auto insurance rates and creating a financial accountability office.
"People get it," said NDP Leader Andrea Horwath. "They realize that the budget was largely the result of NDP ideas that we pushed the government into implementing,"
Henry Jacek, professor of politics at McMaster University in Hamilton, said Hudak needs to do well in these byelections or he'll face grumbling about his leadership after blowing a big lead heading into the 2011 election, and after losing the Kitchener seat they'd held for 20 years in a 2012 byelection.
"I think the big pressure is on the PCs because the Tories are leading the polls in three of the five byelections, so the expectations once again are high for the PCs," said Jacek.
"The history here with Hudak is the polling numbers don’t turn into victories."
The Liberals were one seat short of a majority before five of their members resigned after Wynne became premier last February, and they will still have the most seats in the legislature even if they should lose all five contests.
"On Friday, I will be the premier, and I will continue to do the work of this government and part of that is working with the opposition parties in a minority situation," said Wynne.
The fact the status of the government is not on the line will allow voters to focus more on local candidates and local issues, said Jacek.
"It makes the local candidate far more important than in the general election, where voters think about who will be the government and who they want to be premier or prime minister," he said.
Low voter turnouts are believed to help sitting governments, and both opposition parties were fuming when Wynne scheduled the byelections just before a long weekend at the peak of the summer vacation period.
But the New Democrats predicted it may not work the way the Liberals hope.
"I think it's going to backfire a little bit because I hear everywhere I go that people think this was a cynical ploy," said Horwath.
"So the people who do get out to vote are going to have a bad taste in their mouths because Kathleen Wynne was so cynical in the way she scheduled these byelections."
The Tories haven't won a seat in Toronto since 1999, but hope that will change Thursday in Etobicoke-Lakeshore, where deputy mayor Doug Holyday, the last mayor of the old city of Etobicoke, is running as a Conservative against popular fellow councillor Peter Milczyn for the Liberals.
The Liberals call Holyday a puppet of Mayor Rob Ford, who angered the government by wading into the provincial byelections and saying voting for the Liberals is like giving a bank robber a gun.
It appears the Liberals' efforts to secure a so-called star candidate for London-West, former high school teachers' union boss Ken Coran, backfired when his campaign quickly faltered. The riding is seen as a close two-way between the Tories and NDP.
The New Democrats opened an early lead in Windsor-Tecumseh, former finance minister Dwight Duncan's riding, after they nominated long-time local television reporter and city councillor Percy Hatfield as their candidate. Even Liberals quietly concede they will likely lose the Windsor riding.
The Liberals' best chances Thursday appear to be in McGuinty's old riding of Ottawa-South, although the Tories were doing well there too, and in the other Toronto riding, Scarborough-Guildwood, which is seen as a bellwether for the province.
"Quite frankly I think the Liberals are prepared for bad news, and what gave it away for me was when Wynne announced in June that there would be no election this year," said Jacek.
"They made the decision that the numbers were still so bad that they were ready to commit publicly not to have an election this year."
However, Jacek said the improving U.S. economic performance could spill over into Ontario, boosting jobs and the provincial economy and giving Wynne and the Liberals a much stronger position heading into a general election.