"Does the prime minister think it's business as usual for a senator to defraud taxpayers? Is it business as usual to give a $90,000 payoff?" asked Mulcair.
Standing in for the prime minister, Heritage Minister James Moore blamed the NDP for not supporting the government's plans for Senate reform, which include a mandate for elected-only senators, and a reference about legal questions on term limits and Senate elections to the Supreme Court of Canada.
Harper has not yet answered questions from opposition MPs about the $90,000 cheque written by Nigel Wright as a gift to defray Duffy's improperly claimed Senate expenses. He was on a trade-related trip in South America last week, and does not always attend question period on Mondays.
Moore deflected Mulcair's questions by asking how many of his NDP MPs have not paid their taxes. Last week, it was revealed two NDP MPs, Tyrone Benskin and Hoang Mai, who was once the NDP's revenue critic, owe money in back taxes to the Canada Revenue Agency.
Later in question period, Conservative MP Pierre Poilievre used the words "tax cheats" and "tax evaders," about the NDP.
Justin Trudeau draws return fire
Liberal Leader Justin Trudeau asked if the Conservatives would support a Liberal motion to be put before the ethics committee that would demand Wright and Harper appear as witnesses to explain why Wright paid Duffy's expenses out of his own pocket, and what the prime minister might have known about the payment.
In reply, Moore threw back at Trudeau a remark the Liberal leader made over the weekend. Reading from an article in the French-language newspaper La Presse, Moore quoted Trudeau's remarks.
"He made it so very clear on this weekend that he doesn't believe in Senate reform 'because we have 24 senators in Quebec and there are only six for Alberta and British Columbia. That benefits us. It's an advantage for Quebec.'" Moore said.
"All Canadians should be served by national institutions and the Liberal leader should stop dividing Canadians again and again over these matters."
In an interview with La Presse on Saturday, Trudeau took issue with the NDP's campaign to abolish the Senate, particularly as it could affect his home province of Quebec, where the NDP holds most of its seats.
Trudeau pointed out that Quebec's 24 Senate seats, in comparison with the six Senate seats of each western province, give it weight.
"It's to our advantage. Abolishing it, this is demagoguery," he said in French, referring to Mulcair's campaign.
Trudeau's comments also drew criticism from western premiers. A few hours before question period began, Saskatchewan Premier Brad Wall tweeted, " Disappointed in @JustinTrudeau. He opposes abolition because Senate status quo gives advantage to Que over the west."
Alberta Premier Alison Redford tweeted soon after, "Disappointed by @Justin Trudeau's comments. No need to pit AB/BC against regions. We need elected, equal senate, accountable to Cdns."
Some MPs made an issue of Harper's absence from question period. NDP MP Charlie Angus referred to Harper as "the peekaboo prime minister."
Senate report edited of critical points
Others were critical of the role of Conservative senators in softening the Senate report on Duffy's expenses by removing paragraphs that were critical of his claims his primary residence is in P.E.I.
Duffy repaid $90,000 to the Senate in expenses he'd claimed for what he says he mistakenly considered a secondary residence in Ottawa, using the money he was given by Wright.
NDP MP Pat Martin called the government Senate leader Marjory LeBreton an "artifact of the golden era of Gucci shoes," apparently referring to the fact that she was appointed by former prime minister Brian Mulroney, said to have a closet full of the expensive shoes.
Liberal MP Bob Rae asked how it could be possible that Conservative senators Carolyn Stewart Olsen and David Tkachuk, the two senators who redacted the Duffy report, could now "stand and judge their own behaviour." Stewart Olsen and Tkachuk are on the committee that will supervise a further audit of Duffy's expenses.
Moore replied the government simply doesn't agree with Rae's characterization of Stewart Olsen's and Tkachuk's behaviour, and if the Liberals don't like it, they can complain to the ethics commissioner. The ethics commissioner is already examining the propriety of Wright's cheque to Duffy.
Liberal MP Marc Garneau, responding to reporters in the foyer of the House of Commons, addressed the fact the prime minister is rarely in question period Mondays. "This is not an ordinary Monday. This is a situation where this government has been basically, in the past 12 days, facing its greatest scandal in the seven years that it has been the government."
Conservative MP Mark Wawara told reporters: "Well, the prime minister has a very busy schedule and I believe he will be in the House this week."
Harper is expected to attend question period Tuesday.