TORONTO -- Ontario's opposition parties proposed different strategies Tuesday to get to the bottom of what they say was an attempt by former premier Dalton McGuinty's office to pressure the Speaker of the legislature to change a ruling against the government.

Newly-released emails show senior Liberals in McGuinty's office tried to get Speaker Dave Levac to change his preliminary finding that then-energy minister Chris Bentley was in contempt for not releasing all documents on two cancelled gas plants.

The Progressive Conservatives want the integrity commissioner to find out if the Liberals threatened Levac or made the Speaker an offer to drop the contempt ruling.

"It's absolutely unclear what the threat was or what he was offered or what the demands of him were," said PC energy critic Vic Fedeli.

"I'm looking forward to the integrity commissioner looking into the very integrity of that conversation. What occurred? What was he threatened with?"

The NDP plan to call two of the McGuinty staffers, whose emails talked about pressuring the Speaker to change his contempt finding, to appear before the justice committee hearings into the cancelled gas plants, which resume next Tuesday.

"The first people we need to hear from are those people who were out there trying to bully the speaker," said New Democrat house leader Gilles Bisson. "What we hear from them will determine if we call the Speaker. If need be, we will call the Speaker."

The Tories said discussions about trying to intimidate the Speaker shows the Liberals will stop at nothing to try and bury the scandal over their decision to cancel gas plants in Oakville and Mississauga prior to the 2011 election, which has cost at least $585 million.

"Operatives, including the senior campaign adviser to Premier Kathleen Wynne, clearly attempted to hijack a Speaker's decision," Fedeli wrote in a letter to integrity commissioner Lynn Morrison.

"Which incentives were offered or punishments were threatened is unclear, but these actions threatened and undermine the institutions we value in our democracy."

The NDP said pressing the Speaker in a private meeting to change his ruling is the same as trying to interfere with a judge during a court proceeding.

"That's pretty serious stuff, and akin to somebody going to a judge and trying to influence a judge on a decision," said Bisson. "You don't do that. There are ramifications."

Levac was expected to put out a statement Tuesday responding to the claims he was pressured by top Liberals to drop the contempt ruling.

The emails between senior Liberals showed Levac had rebuffed attempts by former McGuinty aide Dave Gene, who met with the Speaker in an effort to convince him to change the contempt ruling.

McGuinty resigned as premier last September and prorogued the legislature just hours before the committee hearings were to begin into the cancelled gas plants, delaying the inquiry for several months.