Treasury Board President Jane Philpott, a member of Canadian Prime Minister Justin Trudeau’s cabinet, resigned her post on Monday because she “lost confidence” in the administration’s handling of the SNC-Lavalin scandal.

In her resignation letter, Philpott declared herself a true believer in Trudeau’s agenda but said her confidence in his leadership was shaken by former Justice Minister Jody Wilson-Raybould’s testimony that she was improperly pressured to avoid prosecuting engineering giant SNC-Lavalin for corruption.

“The solemn principles at stake are the independence and integrity of our justice system. It is a fundamental doctrine of the rule of law that our Attorney General should not be subjected to political pressure or interference regarding the exercise of her prosecutorial discretion in criminal cases,” Philpott wrote.

Perhaps the most cutting line in Philpott’s letter of resignation was when she declared, “There can be a cost to acting on one’s principles, but there is a bigger cost to abandoning them.”

Wilson-Raybould herself resigned abruptly in February. She hailed Philpott as “incomparable” on Monday and saluted her “constant and unassailable commitment to always doing what is right and best for Canadians.”

Trudeau’s top aide and close friend Gerald Butts resigned later in February, insisting there was no wrongdoing in the effort to protect SNC-Lavalin from prosecution, but he will return to the spotlight on Wednesday to testify before the House of Commons Justice Committee.

The Toronto Sun on Monday judged Butts may have become the key figure in the battle for Trudeau’s political survival, saying:

The most damning evidence put forward by Wilson-Raybould is from two text messages her chief of staff, Jessica Prince, sent to her following meetings she had with the PMO [Prime Minister’s Office]. Butts apparently told her “there is no solution here that does not involve some interference.” Trudeau Chief of Staff Katie Telford apparently also told her “we don’t want to debate legalities anymore.” Both of these — the Butts one in particular — suggest they knew what they were doing was not in line with the law, yet didn’t care. The Conservative and NDP MPs on the committee will need to grill Butts on these issues. Canadians want answers.

In other words, Butts’ testimony may determine whether Trudeau’s efforts to protect SNC-Lavalin are upgraded from “inappropriate” to “illegal.”

Trudeau’s defense strategy appears to depend on carefully chipping away at Wilson-Raybould’s credibility without flatly accusing her of lying, a strategy that will presumably widen to include Philpott as well. According to the latest polls, it’s not working – Trudeau’s numbers are collapsing, Canadians say they are following the story closely, and some 67 percent of them believe Wilson-Raybould instead of Trudeau.