PETERBOROUGH, Ont. - A contentious plan to harmonize the provincial sales tax with the federal GST could haunt the governing Liberals in an upcoming Toronto byelection, Premier Dalton McGuinty acknowledged Wednesday.

While he's proud of his government's policies, voters in the riding of St. Paul's may not agree when they head to the polls Sept. 17, he said.

"You can never tell what's going to happen in a byelection," McGuinty said in Peterborough, Ont., after announcing funds to widen and improve a 5.6-kilometre stretch of Hwy. 7.

He then turned to local Liberal Rick Johnson -- whose upset victory over former Progressive Conservative leader John Tory in a March byelection caught even the premier by surprise -- and quipped, "Exhibit A."

"Like the leader of all the parties, you want to win all of them. You do the best that you can," McGuinty added.

"I think it's well understood that it's harder in government to win a byelection than it is in opposition."

The byelection, which was called Wednesday, will fill the provincial seat vacated in June by his former cabinet minister Michael Bryant who held the riding for a decade.

It will be the first time voters have gone to the polls since the Liberal government unveiled its controversial plan to move to a single sales tax next July.

The move, which British Columbia is also pursuing, proved to be politically toxic for some provinces when consumers ended up paying more for items that were previously exempt from the provincial levy.

The Progressive Conservatives are already planning to put the costs of tax harmonization front-and-centre during the byelection campaign.

The tax change will likely be discussed along with other pressing issues, such as Ontario's ailing economy, health care, education and the environment, McGuinty said.

But moving to a single 13 per cent tax will help the province become more competitive in the long run, he said.

"It's not the kind of thing that I expect ... would cause Ontarians to lift me onto their shoulders so that we might parade up and down the streets together," he said.

"I think what we need to ask ourselves is, 'What do we need to do?' Not just for our generation. What do we need to do for our kids? What do we need to do to ensure that we have a stronger economy?"

The Liberals hope to ease consumer pain and minimize the political fallout by offering cheques of up to $1,000 to families and individuals when the change takes effect next July.

The last cheques will be in mailboxes just a few months before the next provincial election in October 2011.

Dr. Eric Hoskins, who was an adviser to former federal cabinet minister Lloyd Axworthy, will run on the Liberal ticket.

Toronto newspaper columnist Sue-Ann Levy is expected to represent the Tories following a nomination meeting Thursday.

The NDP haven't yet chosen their candidate. Lawyer Julian Heller, former B.C. Green Party leader Stuart Parker and former provincial politician Dr. Bob Frankford are all vying to represent the party.

Realtor and community activist Chris Chopik will run for the Green Party.

Speculation has been swirling that McGuinty delayed the byelection to give his party an opportunity to find a candidate who could fill Bryant's shoes.

But McGuinty said he was just trying to give all the parties -- and voters -- enough time to prepare for the race.

"I wasn't a big fan of plunging us into one in the middle of the summer, because I think that's not fair to voters whose attention is often turned elsewhere during the holiday season," he said.

Nova Scotia, New Brunswick and Newfoundland and Labrador harmonized their sales taxes with the federal GST more than 10 years ago. Quebec partially harmonized its sales tax system and Alberta has no provincial sales tax.