The Conservatives are piling pressure on five Liberal MPs who will determine today whether a House of Commons committee will investigate an allegation that former attorney general Jody Wilson-Raybould was pressured to help SNC-Lavalin avoid criminal prosecution.

Conservative deputy leader Lisa Raitt says if Liberals believe Prime Minister Justin Trudeau's insistence that the allegation is false, they'll support "without hesitation" an opposition motion calling for the justice committee to hear from nine senior players in the government, including Wilson-Raybould herself, her replacement in the justice portfolio, David Lametti, and top staffers in the Prime Minister's Office.

"But if they defeat or they water it down in any way, it is nothing less than an admission of guilt," Raitt said Wednesday at a news conference held just two hours before the justice committee is scheduled to meet to consider the motion.

Conservative Leader Andrew Scheer, who is touring New Brunswick, repeated that message a short time later.

"The truth is not partisan," he said.

Liberal MP Randy Boissonnault began the meeting of the Commons justice committee by proposing a list of three witnesses: Lametti, Privy Council clerk Michael Wernick, and the deputy minister of justice.

They could testify on the Shawcross Doctrine, the rules about how the attorney general should conduct him- or herself to keep law enforcement and politics separate, committee chair Anthony Housefather said, suggesting the three names are just a start.

The New Democrats' Nathan Cullen pointed out that Wilson-Raybould is not on the Liberals' list and her account is likely the most important.

Wilson-Raybould resigned from the federal cabinet Tuesday, but gave no reason publicly. In a statement, she said she has hired a former Supreme Court justice, Thomas Cromwell, to advise her on how much she can say without violating solicitor-client privilege.

Trudeau has denied Wilson-Raybould was pressured last fall to instruct the director of public prosecutions to negotiate a remediation agreement with SNC-Lavalin rather than pursue a criminal trial on charges of bribery and fraud related to the company's efforts to secure government contracts in Libya.

He went on the offensive Tuesday, saying Wilson-Raybould had an obligation to come to him if she felt she had been improperly pressured. Instead, the allegation leaked out from anonymous sources last week, months after the alleged pressure and one month after Trudeau shuffled Wilson-Raybould to the veterans affairs portfolio, widely seen as a demotion.

Scheer accused Trudeau of "trying to paint himself as the victim in all of this," while "publicly impugning (Wilson-Raybould's) character in a way that prevents her from speaking for herself."

The governing Liberals, who hold the majority on the justice committee, appear open to conducting an investigation, but may balk at the witness list proposed jointly by the Conservatives and New Democrats.

Liberal members met before the meeting to discuss their approach. Housefather, a Montreal Liberal MP, said he had not consulted with the Prime Minister's Office.

Cullen called Trudeau's performance Tuesday "reprehensible," particularly that he called Wilson-Raybould "Jody" several times.

Raitt said the Conservatives will pursue other options if the Liberals block or water down a committee inquiry. Among the options, she mentioned a judicial inquiry or an investigation by a Senate committee.

She did not rule out asking the RCMP to investigate.

"It all starts today with the parliamentary committee," Raitt said.

"It's in the hands of those five Liberals to determine whether or not we're going through the committee process and have the appropriate questions asked and the answers given so we can get to the bottom of the matter for Canadians."

Federal ethics commissioner Mario Dion has initiated his own investigation into the matter, specifically whether there's been a violation of the Conflict of Interest Act.

In the wake of the allegations and Dion's review, SNC-Lavalin had its debt rating downgraded Wednesday by Standard & Poor's to BBB- from BBB.

The agency cited the criminal charges against the engineering and construction giant and the possibility of a 10-year ban from bidding on federal contracts among its reasons for the downgrade.