In an open letter to Liberal Premier Kathleen Wynne and Progressive Conservative Leader Tim Hudak, Horwath says Ontarians "deserve the facts" before they go to the polls on June 12.
"Everybody that I talk to says they're tired of the corruption, they don't want to see the corruption continue at Queen's Park. That's why I sent a letter today to Mr. Hudak and Ms. Wynne asking them both to join me in asking the Speaker to release the documents that the OPP are asking for," Horwath said Saturday.
"The way that Ms. Wynne has handled this situation — that's not what leadership is. What leadership is is doing everything you possibly can, even if that is sending a letter suggesting that what you want is that the documents get released."
The gas plants became a focal point on the campaign trail this week after police investigating the alleged destruction of government documents related to the plant cancellations asked for key records from the legislature.
Horwath said those are the same documents the NDP tried to have released through a justice committee probing the cancellations, but she said Liberals on that committee blocked that request.
The two plants were cancelled under Wynne's predecessor, Dalton McGuinty, and are estimated to have cost taxpayers up to $1.1 billion in what opposition parties have said was a move to save Liberal seats.
Horwath said Wynne can't run from the Liberal record of scandals and "corruption," which she said is leading to a growing number of former Liberal supporters turning to the NDP.
"This is not a choice between bad ethics and bad math," Horwath said, taking a swing at job projection figures that have been questioned in the Tory platform.
"That ballot box decision that people will make will be to vote against corruption and to vote for a plan that makes sense and focuses on their priorities."
Wynne shot back at Horwath on Saturday, saying the NDP leader was simply trying to distract voters from the lack of a coherent plan from the New Democrats.
"She doesn't want to talk about what she believes she can do, because she hasn't said what she stands for," Wynne said.