"Does the prime minister agree that there was a criminal coverup organized out of his own office?" NDP Leader Thomas Mulcair said in the House, making reference to the version of events laid out for the media by Harper's spokesman over the weekend.
Paul Calandra, Harper's parliamentary secretary, said repeatedly that only Nigel Wright and Duffy are the subject of an RCMP investigation into the Senate scandal, not the prime minister.
He also returned fire to Liberals with frequent references to the sponsorship scandal of almost two decades ago under former prime minister Jean Chretien.
Jason MacDonald, the prime minister's spokesman, was in touch with several media outlets Sunday, a few days after the release of an explosive RCMP affidavit on the scandal. The document contained allegations casting doubt upon the prime minister's long-held insistence that he knew nothing about Wright's plan to cut Duffy a $90,000 cheque to cover his ineligible expenses.
The RCMP affidavit cites an email from Wright to staff saying "we are good to go from the PM" after the two men sat down to discuss what to do about Duffy last February.
But MacDonald said that meeting consisted simply of a conversation about compelling Duffy to repay his expenses — estimated then to be about $32,000 — and that there was no plot hatched to have Wright write the cheque, nor to whitewash a Senate report on the disgraced senator.
As MacDonald did a day earlier, Calandra denied that Harper had any knowledge that Sen. Irving Gerstein, chairman of the Conservative Fund Canada, planned to use party funds to pay Duffy's expenses.
Nor did the prime minister know, Calandra said, that Gerstein contacted a partner at Deloitte Canada about its Senate-ordered audit of Duffy's expenses in an apparent attempt to have the accounting firm drop the senator from the probe, according to the RCMP allegations. Those accusations have yet to be tested in court.
The RCMP also alleges the Prime Minister’s Office argued that Duffy's transgressions would become moot once the senator repaid his expense claims, and so he should be removed from the audit.
Gerstein, the Conservatives' self-styled chief bagman, hasn't responded publicly to those charges as the opposition howls for his removal from the Conservative caucus.
The notion that Harper was completely in the dark about an alleged coverup involving his chief of staff, as well as Gerstein, was mocked once again by Opposition members on Monday, the same day that voters in four federal ridings were heading to the polls to cast their ballots in a series of key byelections.
"Mr. Harper keeps trying to say the person who's solely responsible is Nigel Wright, but the head of the Conservative fund was offering to do the exact same thing; that's obviously a key problem here," Mulcair said outside the House.
"Gerstein was planning to do the exact same thing (as Wright) that is now being investigated and reproached of him, so why is Irving Gerstein still sitting in the Conservative caucus?"
Democracy Watch, a non-profit organization devoted to holding governments accountable, accused Gerstein on Monday of violating at least two sections of the Senate's conflict-of-interest guidelines. It called on the Senate ethics officer to launch an inquiry into Gerstein's actions.
The Senate ethics officer did not reply for a request for comment.
The Senate internal economy committee has asked Deloitte officials to come to Parliament Hill later this week to discuss their contact with Gerstein. Neither Conservative nor Liberal senators would comment Monday on whether that meeting would be open to the public.
"Sen. Gerstein tried to use his influence to have the audit changed, and to obtain and convey information about it, which is exactly what Deloitte's spokesperson has said is improper," Duff Conacher, founding director of Democracy Watch, said Monday.
Note to readers: This is a corrected story. An earlier version said three byelections were being held Monday.