Campaigning in the border city of Sarnia, Horwath railed against her rivals as she pledged an NDP government would get soaring hydro rates down.
Currently, power rates are projected to rise by about 40 per cent over the next four years, with the province's industries paying more than double that charged in neighbouring jurisdictions.
"This mess started with the Conservatives, when they began to deregulate and privatize our electricity system," Horwath said.
"For the last 10 years or so, the problems that the Conservatives started have just been entrenched by the Liberals."
In an effort to lower "jaw-dropping" hydro bills, the NDP is pledging, if elected June 12, to merge four of the five agencies that run the power system in Ontario and cap the salaries of the CEOs in charge.
The party, if elected, would end subsidized power exports. It is also promising to scrap the provincial sales tax on residential hydro — worth about $120 a year for the average rate payer.
Horwath said affordable hydro was once one of the province's strong points.
"This beast used to be a beast that was of benefit to Ontario's economy — we used to have rates that were very competitive," she said.
She did not mention that power rates were kept artificially low for years, or that cost-overruns on building nuclear reactors contributed to the province's huge hydro debt.
Horwath also ramped up her attacks on her rivals, Liberal Premier Kathleen Wynne and Progressive Conservative Leader Tim Hudak.
"The Liberals, let's face it, are a scandal-ridden, wasteful government that has become out of touch with Ontarians that people tell me they simply don't trust any more," Horwath said.
"People are (also) very wary of a Tim Hudak Conservative government that would throw 100,000 people onto the unemployment line."
In response, Hudak called the NDP the "great pretenders," saying they voted with the Liberals to increase hydro rates.
The Tories are promising to lower prices by importing energy from Quebec and the United States, cutting bureaucracy at Ontario Power Generation and Hydro One, and ending subsidies for wind and solar power.
Horwath repeated her message at a rally in London, attended by about 40 people, where she promised the NDP would release its platform "very, very shortly."
She then headed to Cambridge and Kitchener as part of a busy, day-long swing through southwestern Ontario.