Only a change of government will bring transparency and accountability to Queen's Park after years of Liberal scandals, Horwath said.
The NDP leader has repeatedly blasted the Liberals for cancelling two gas plants — including one in the days before the 2011 election — a move that could eventually cost the province up to $1.1 billion.
At a campaign stop in Toronto, Horwath said it's clear the Liberals "used tax dollars inappropriately" to further their political ambitions — and Wynne, who was a senior cabinet member at the time, signed off on the decision.
Asked if she thought Wynne herself was corrupt, Horwath would only say the premier was "at the centre" of the scandal and "the behaviour speaks for itself."
She doubled down later while campaigning in St. Catharines.
"It is a party rife with corruption, I think that is obvious," she said.
"And regardless of how they change the chairs at the cabinet table it continues to be the same corrupt Liberal party that brought us the gas plants, that brought us eHealth, that brought us Ornge air ambulance."
Wynne fired back, saying Horwath is "slinging mud" because she can't justify the NDP's decision to force the June 12 election.
"She doesn't know why she rejected the budget," the premier said in Sault Ste. Marie. "It's where she has gone, making false accusations she knows aren't true."
The Liberals also issued a statement calling the accusations an attempt to distract from the NDP's "failing" election campaign.
Wynne is suing Progressive Conservative Leader Tim Hudak over comments he made in March suggesting she "possibly ordered'' the alleged destruction of government documents related to the gas plants scandal.
But Horwath brushed off concerns that she could face a lawsuit, saying she is more worried about ensuring Ontarians have "a government they can actually trust."
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