Ontario's Liberals and Progressive Conservatives are virtually tied in popularity with voters, while approval for the New Democrats and Leader Andrea Horwath has declined this summer, according to a poll to be released Thursday.

The new Nanos Research poll shows the Conservatives at 34.7 per cent and the Liberals at 34 per cent after 1,000 Ontarians were asked between Aug. 11 and 16 who they would vote for locally. The NDP stood at 22.1 per cent, down 6 percentage points from May when a similar survey was conducted.

Ms. Horwath's leadership score – based on responses about trust, competence and vision for Ontario – also dropped by 6 percentage points. Scores for Premier Dalton McGuinty and Opposition Leader Tim Hudak both went up.

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"The narrative over the last while has really focused on Dalton McGuinty and Tim Hudak fighting things out," said pollster Nik Nanos. "As a result, it seems like Andrea Horwath has not had as much profile and those NDP numbers have gone down."

Mr. Nanos said Ms. Horwath needs to be very careful about when she co-operates with the minority government because her party typically does better when it offers a strong opposition.

It's the first time in a year that there's been a softening in support for the NDP and its leader, Mr. Nanos said. "We have to see whether this is the new trend for her or this is just a one-off low period," he said.

At the same time, Mr. Nanos said Mr. Hudak been "aggressively asserting himself" on cutting costs and managing government spending.

He said this may have driven some "nervous New Democrats into the Liberal camp," especially with two by-elections coming up next month in Vaughan and Kitchener-Waterloo, giving the Liberals a chance at a majority government. The survey by telephone is considered accurate plus or minus 3.1 percentage points, 19 times out of 20.

The poll's respondents were asked which party leader they trust the most to manage six issues. Mr. McGuinty and Mr. Hudak were tied in first when it came to health care. Mr. Nanos said it's "very unusual" for the PCs Progressive Conservatives to do as well in that area and probably reflects dissatisfaction with the way the Liberals have handled health issues.

A year ago, 33 per cent of people polled said Mr. McGuinty was the most trustworthy when it came to education issues. This month, the percentage dropped to 24 per cent amid the deterioration of his relationship with teachers over contract negotiations.