Liberal Sean Casey sent a letter Thursday to RCMP commissioner Robert Paulson asking police to look into whether someone in the party promised to pay off Duffy's expenses.
Casey points to an RCMP affidavit filed in court last week, which suggests Conservative Fund Canada chairman Sen. Irving Gerstein was prepared at one point to use party funds to repay Duffy.
"He stated that it is something he would consider," RCMP Cpl. Greg Horton said following an interview with Gerstein in September.
According to the documents, the alleged agreement included a condition that once Duffy repaid the expenses, he would be reimbursed by a third party. Ultimately, Nigel Wright — Prime Minister Stephen Harper's chief of staff at the time — paid out of his own pocket when the bill — $90,000 — got too high for the party to bear.
The Liberals point to section 16 of the Parliament of Canada Act, which makes it illegal to give or promise to give a parliamentarian money in an attempt to influence them.
It notes that the influence could relate to any "...controversy, charge, accusation...or other matter before the Senate or the House of Commons or a committee of either House."
"It seems to be you've got a promise to pay a senator in relation to a controversy which is properly before a committee. It seems rather simple," said deputy Liberal leader Dominic LeBlanc.
Gerstein did not return calls to his office Friday.
Casey pressed his case Friday during question period in the Commons.
"Could the attorney general explain why he has not asked the RCMP to investigate the actions of Sen. Gerstein and certain former PMO staffers, as I have?" he demanded.
"Why do I have to do his job for him, and what are they hiding over there?"
Paul Calandra, Harper's parliamentary secretary, emphasized that the RCMP have been investigating the Duffy repayment matter for months.
"The RCMP have put in a number of documents, which have been referred to by the opposition on a number of occasion," Calandra said.
"Those same documents show that the RCMP are investigating Nigel Wright and Sen. Duffy; Nigel Wright for repaying expenses that Sen. Duffy accepted but did not incur. Those are the people who are under investigation at this time."
The Conservative party did wind up covering $13,000 of legal fees Duffy incurred while his living expenses were under scrutiny.
On Friday afternoon, Harper's Twitter feed posted a link to a "quick recap" of what he has been doing over the past couple of weeks, including attending the Grey Cup game in Regina and supporting the victims of Typhoon Haiyan in the Philippines. (https://twitter.com/pmharper). There was no mention of the Senate spending scandal that has dominated question period.
Also on HuffPost