Voters in two Ontario ridings head to the polls Thursday in a pair of provincial byelections that could reshape the political landscape at Queen’s Park.

Liberal wins in the riding of Kitchener-Waterloo and Vaughan would give Premier Dalton McGuinty the 54 seats needed to hold a majority in the 107-seat legislature. The Progressive Conservatives enter Thursday’s byelections with 36 seats while the NDP holds 17.

Polls in both ridings will remain open until 9 p.m. ET.

Most observers expect the Liberals will hold their seat in Vaughan, a fast-growing riding north of Toronto. Liberal Greg Sorbara held it for 20 of the past 22 years before quitting last month to focus on preparing the Liberals for the next provincial election.

The real fight is expected in Kitchener-Waterloo, where a three-way race is emerging between the major parties. Barry Kay, a political science professor at Wilfrid Laurier University, said it's rare to see such high stakes in a byelection vote.

"I don't know of a parallel situation in Ontario's history ... where one byelection could make the difference between a majority and a minority government," he told The Canadian Press.

McGuinty triggered the Kitchener-Waterloo byelection by convincing veteran Progressive Conservative Elizabeth Witmer to give up her seat to become chair of the Workplace Safety and Insurance Board. The riding is a traditional Tory stronghold — Witmer held it for 22 years — but the NDP has made gains in the area following McGuinty’s move to legislate a wage freeze for teachers.

Voters in Kitchener-Waterloo appear to have been turned off by McGuinty's attempts to get a majority, said Kay.

"I thought that would play better than it has," he said. "I think they are disinclined to give the party a majority. It's not just a neutral factor. I think it's a negative factor."

Voter turnout is key

McGuinty has said wins in both ridings could be a boost to the Liberals as they work to pare back a $15-billion deficit and recover from the Ornge air-ambulance scandal.

"It's always a little bit easier when you have a majority to act on a mandate whether you're trying to introduce a budget or move ahead with a putting students first act,” he told CBC News.

PC Leader Tim Hudak said voters are fed up with the Liberal record on fiscal issues, saying the province has gone deeper into debt and lost more jobs since last fall’s provincial election.

"People are shaking their heads and saying ‘Now he wants a majority?’" Hudak said.

NDP Leader Andrea Horwath said the Liberals set off the Kitchener-Waterloo byelection to distract voters from Ornge and other problems.

"It was a very cynical ploy that people recognized for what it was, and I think voters of Kitchener-Waterloo were really disappointed that the government decided to go in that direction instead of addressing the real issues they have there," she told The Canadian Press.

Luisa D'Amato, a columnist with the Waterloo Region Record newspaper, said the Kitchener-Waterloo byelection will likely come down to voter turnout.

"What it will really depend on is how good each campaign gets out their vote,” she said. “They understand this is a big responsibility, that what they decide will affect the whole province.”

With files from The Canadian Press