Ontario Premier Dalton McGuinty is threatening to call a snap election, blaming the New Democrats for backtracking on a pledge to prop up his minority Liberal government.

Mr. McGuinty is accusing NDP Leader Andrea Horwath of "breaking her word" and joining forces with the Progressive Conservatives to "gut" his government's budget bill.

This is not the first time the short-lived marriage between the Liberals and NDP has been in jeopardy. But with the vote on the omnibus budget bill set for next Wednesday, the stakes are much higher.

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"It's absolutely imperative that we pass this budget," Mr. McGuinty said in a statement on Thursday. "And if we cannot pass this budget, we will take it to the people in a general election."

Ms. Horwath struck a more conciliatory tone, noting that she made a commitment to ensure that the budget bill passes and that she intends to keep her word.

"I hope the Premier sees reason and doesn't call an election no one wants," she said in a statement.

Officials from all three parties at the Ontario legislature held a series of late-day news conferences following clause-by-clause debate on the budget bill Thursday afternoon. Tory and NDP members on the standing committee on finance voted against entire sections of the bill. The Liberal MPP chairing the hearings called a 20-minute recess after each vote.

"They're now realizing that they don't have a majority government. They can't ram this thing through," John O'Toole, a Tory MPP on the committee, told reporters.

Finance Minister Dwight Duncan said the opposition changes to the budget bill undermine the government's ability to execute its fiscal plan, including erasing the province's $15-billion deficit by 2017-18.

If the NDP does not agree to restore the deleted sections, Mr. Duncan told reporters, the Premier will inform the Lieutenant-Governor next Tuesday that his government no longer has the confidence of the legislature – a move that would force an election around mid-July.

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Without the deleted sections, Mr. Duncan said, "it's not our budget."

In a bid to save their minority government, the Liberals struck an accord with the NDP last April by adding major concessions in the budget bill, including a new surtax on the rich and a freeze on corporate tax rates.

Ms. Horwath pushed for the changes in return for supporting the budget. But she has said the 350-page budget bill contains many measures that the New Democrats have problems with and that need more scrutiny. She cited provisions in the bill that would give the Liberals the power to create more entities like the scandal-plagued Ornge air ambulance service to handle government services.

The NDP also had problems with proposed changes to the Endangered Species Act in the bill that would exempt private landowners from taking measures to protect wildlife or habitat. The NDP voted with the Tories on Wednesday to delete the environmental provisions.

"Nothing that's happened today should surprise the government," NDP House Leader Gilles Bisson told reporters. But he declined to say whether the NDP plans to reinstate any of the deleted sections.

Mr. Duncan also took aim at the Tories for voting against arbitration measures in the bill that were part of their own platform for the election campaign last October.

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"I've never seen anything like this," Mr. Duncan said.