In a letter to NDP Leader Andrea Horwath obtained by media outlets including The Canadian Press, 34 long-time supporters say they are "seriously considering" not voting for the New Democrats on June 12.
The writers _ who include Michelle Landsberg, wife of former Ontario NDP leader Stephen Lewis _ accuse Horwath of pushing the party "to the right of the Liberals in an attempt to win Conservative votes."
They urge her to reconsider her stance or risk losing the support of thousands "who will turn to other parties or not vote at all."
The letter, dated Friday, comes a day after the New Democrats unveiled a platform that would raise the corporate tax rate and offer financial help to hydro users, family caregivers and students if they form government.
In a statement issued late Friday, the New Democrats defended their decision to trigger an election and said they stand by their platform.
"Progressives cannot turn a blind eye to corruption because it weakens the very foundations of our democracy. We need to renew democracy by guaranteeing transparency and accountability," campaign co-chair Gilles Bisson said.
"The support our leader is getting from people has never been warmer, our ranks never been stronger. We’re proud to stand for raising the minimum wage, increasing childcare spaces, lowering tuition, and supporting caregivers."
Earlier Friday, Horwath brushed off suggestions the party could lose ground to vote splitting among left-leaning voters eager to stave off a Tory government.
The NDP leader said she doesn't see Tim Hudak's plan — which includes cutting 100,000 public sector jobs — gaining enough traction to warrant strategic voting come June 12.
And people should know better than vote Liberal by default, she said during a campaign stop in Ottawa-Vanier, a long-held Liberal riding.
"I do not believe that it is inevitable that you have to vote Liberal and reward them for their bad behaviour," she said.
"I think people have decided that it's time to take a hard look at what they want in government and what they want is a government that they can trust to actually deliver for them."
As for Hudak's plan to create a million jobs over the next eight years while trimming the public service, "I don't think anybody takes that plan very seriously at all," the NDP leader said.
Both the Liberals and the Tories have tried to relegate the third party to fringe status, painting themselves as the only two options to lead the province.
But Horwath said she believes Ontarians will vote for whoever will serve them best, rather than "a particular political party that would have you think that they're the only choice."
The Liberals have criticized the plan, which they said would jeopardize Ontario's position as the top jurisdiction in North America for direct foreign investment.
Premier Kathleen Wynne also accused the NDP of cribbing part of the Liberals' spring budget — the same blueprint they rejected, triggering the election.
Horwath fired back at the premier Friday, saying Wynne must not have read the NDP platform carefully.
If there are any similarities, she said, it's because "some of the things the Liberals threw into their budget are NDP ideas as well."
Also on HuffPost