Ontario NDP Leader Andrea Horwath is dismissing suggestions she's alienating core supporters with her attempts to make the party more business-friendly — and electable.

An open letter signed by 34 current and former New Democrats — some well-known within the party — accused her Friday of sacrificing progressive values to court Conservative votes.

The letter-writers said they were "angry" Horwath did not support the Liberal budget on May 1, calling it "the most progressive budget in recent Ontario history."

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Ontario NDP Leader Andrea Horwath's decision to force an election over the budget has come under fire from party supporters. (Adrian Wyld/Canadian Press)

"From what we can see you are running to the right of the Liberals in an attempt to win Conservative votes," the letter reads. "It is not clear whether you have given up on progressive voters or you are taking them for granted."

The group went on to say that they were "seriously considering not voting NDP" this time.

Asked about the letter at a campaign stop in Windsor, Horwath would only say her party is "very democratic" and "people have a right to voice their opinions."

"There's no doubt that I had a difficult decision to make a couple of weeks ago when I decided not to support the budget, but look if there's one straightforward, upfront job that progressives have it's to shut down corruption and get rid of the kind of behaviour we've seen from the Liberal government," Horwath said in Windsor to cheers from supporters.

"The cover-ups, those things are not acceptable because they erode our very democracy. So I made a decision because I think it was in the best interest of the province, and now, frankly, it's up to the people of this province to make a decision."

The warning from the party insiders came a day after the NDP platform was released and during a campaign where Horwath's decision to force the election has been criticized by NDP insiders such as Gerry Caplan, along with Liberals.

Some political analysts say the NDP needs to work quickly to try to convince traditional New Democrats that Horwath is not abandoning the party's principles to get into power.

"They're upset that the NDP is talking about the middle class not about the working class," said University of Toronto political science professor Nelson Wiseman.

"You can see it in some NDP circles where people are not terribly enthused."

Prof. Henry Jacek of McMaster University in Hamilton said Horwath angered some NDP supporters by offering tax cuts to small businesses to offset a promised hike in the minimum wage to $12 an hour. They also didn't approve of her decision to purchased a front-page 'wrap-around' ad in the Toronto Sun, which is not known for its support of New Democrat values, he added.

"I've got a lot of calls from New Democrats who are really upset at (her) trying to go after people I'd call working class Tories, the people who read the Toronto Sun," said Jacek. "I see New Democrats asking 'Why the hell is she advertising in the Toronto Sun?' That's a right-wing, neo-conservative, populist newspaper, so that's confused a lot of traditional New Democrats."

Horwath's decision to trigger an election also did not sit well with some union leaders, who considered the Liberal budget very labour- and NDP-friendly.

with files from CBC News