(Adds background, quotes from NDP leader, paragraphs 3-8)

By Leah Schnurr

TORONTO, May 2 (Reuters) - Ontario's opposition New Democrat Party leader said on Friday her party would not vote to support the budget proposed by the minority Liberal government, likely setting up Canada's most populous province for a near-term election.

"We do not support this government any longer, so we will not be voting in favor of any confidence motion coming forward, including the budget," the NDP's Andrea Horwath told reporters.

With just 48 seats in the 107-seat Ontario legislature, Premier Kathleen Wynne's Liberals need the backing of at least one opposition party to pass the budget and avert an election. The NDP propped up the Liberals and helped pass the government's budget last year.

Horwath accused the Liberals of failing to live up to promises to cut auto insurance rates, create a provincial financial accountability officer and improve home healthcare services.

"The same government that couldn't fill these three promises in the last year is making more than 70 new promises this year," she said.

Ontario's government released a budget on Thursday that projected a larger-than-forecast deficit for 2014-15.

The budget included a proposal for a mandatory provincial pension plan and a tax increase for high-income workers, as well as spending for transportation and public infrastructure and social services.

The right-leaning Progressive Conservatives, who hold the second-largest number of seats in Ontario's legislature, have denounced the budget as a "charade." (Reporting by Leah Schnurr and Cameron French; Editing by James Dalgleish and Tom Brown)