"People are feeling ignored by the provincial government, they're feeling ignored by the Liberals and they feel that they've been taken for granted," she said.
"With this situation now that the premier is not even coming to northern Ontario for the debate, I can't imagine they'd be anything more than extremely disappointed at the least, and probably quite angry and insulted."
Horwath has been pushing for a debate in the north for weeks, saying issues such as unemployment and the high cost of living in that area deserve their own turn in the spotlight.
Progressive Conservative Leader Tim Hudak also accepted the debate invitation from the Northwestern Ontario Municipal Association, but McGuinty declined, citing a scheduling conflict.
The opposition leaders have been ratcheting up the pressure on McGuinty to join them in Thunder Bay and justify the expensive energy policies and high taxes that they say are hurting the region.
The debate will also serve as a dress rehearsal for the two rookie leaders, who will face off against McGuinty in the televised leaders' debate in Toronto next Tuesday.
The veteran Liberal leader denied he was snubbing the north, saying his track record over the last eight years show a "continuing commitment" to the region.
"It's really important in the context of the debate that we're going to have together that we find opportunities to speak to all Ontarians," McGuinty said.
"You wouldn't be surprised if I got invitations to have a GTA-specific debate, maybe a French-language debate, maybe something that's more specific to Eastern Ontario ... I think we need to have one fullsome debate and an opportunity to speak to all Ontarians about the issues that concern them."
The Liberals held six northern ridings when the election was called, including those of two cabinet ministers: Sudbury's Rick Bartolucci and Thunder Bay's Michael Gravelle. The NDP hold three, but are looking to expand their territory.
They're also hoping to build on the gains made by their federal cousins in the May 2 national election, which saw the party grab 22 seats in Ontario and vault into the Opposition benches. The provincial party held 10 seats when the election was called.
Recent public opinion polls suggest there's a tight race developing between the Liberals and Conservatives, with the NDP close behind in third. That could make Horwath the kingmaker if a minority government is elected.
But the NDP leader is shying away from suggestions that she'll back the Liberals if that happens.
Horwath said she'll wait for voters to make their decision and won't pre-judge the outcome of the vote.
"This isn't about me. It's not about how much power I get," she said during a visit to an elderly woman's home in north Toronto to talk about home care.
"It's about how can we deliver the kind of change that Ontarians deserve — the kind of change that puts people first."
But she wouldn't rule out an alliance.
"I'm not ruling anything out because the people haven't made their ruling yet," Horwath said. "And when the people make their ruling, then we'll talk about how it is that we deliver for them."
Her comments come after her interview with the Toronto Star editorial board, where she hailed the progress of an NDP-backed Liberal government in the mid-1980s.
Hudak, who was also campaigning in Toronto, also shrugged off questions about who he'd make a deal with if voters don't deliver a majority.
"If you're looking for someone who's going to do the backroom deals, who's going to do the secret agreements, that's Dalton McGuinty, that's not me," he said.
"I'm actually going to do something unique here ... I'll let the voters decide."