The Senate expenses scandal returned to centre stage in Parliament Monday, with opposition MPs making it the focus of the House of Commons question period.

Prime Minister Stephen Harper was back in Ottawa after his trip to South America last week but was not in the Commons, as is his custom on Mondays.

Nevertheless, Opposition leader Tom Mulcair led off with pointed questions on the Senate expenses and the $90,000 personal cheque the prime minister's former chief of staff, Nigel Wright, gave to Mike Duffy to repay his ineligible expenses.

"Does the prime minister think it is business as usual for a senator to defraud taxpayers? Is it business as usual to give a $90,000 payoff? Dodging questions about political payoffs was shameful when [former prime minister] Paul Martin did it, why does the prime minister think it is just business as usual today?" Mulcair said.

Heritage Minister James Moore, standing in for the prime minister, responded that the Conservatives have tried to pass legislation to reform the Senate and bring in Senate elections but have been blocked by the opposition.

Marjory LeBreton, the government leader in the Senate, made the media rounds Sunday, telling CBC News that Liberal senators must pass new rules governing Senate expenses and travel before the summer, "otherwise the Senate as an institution cannot survive."

Senate Opposition leader James Cowan said the Conservatives are trying to deflect attention from Duffy and Wright.

According to Cowan, the Liberals in the Senate agreed to pass the new rules last week.

"The Conservatives have the majority in the Senate. They could have passed it last Wednesday if they wanted to, they could pass it next Tuesday if they want to. It's up to them. It's not up to us. We're not delaying anything."

Talk of abolishing the Senate and passing what most see as "minor" rule changes is "a desperate attempt by them to change the channel," Cowan said.

Harper has said he did not know about Wright's "gift" to Duffy beforehand, and denied the government is keeping documents about the arrangement from the opposition.

"I'm not aware of any formal agreement on this," Harper said last Thursday during a press conference in Cali, Colombia.

LeBreton also pleaded ignorance on Sunday. 

"As far as I know, there's no documents or there was no paper trail," LeBreton said.

Ethics motion on Monday's agenda

The Commons ethics committee is scheduled to meet on Monday afternoon, with a motion from Liberal MP Scott Andrews on its agenda calling for hearings on the Wright-Duffy repayment affair.

Such hearings could include calling Harper himself as a witness. But Tories may use their majority on the committee to vote the motion down.

NDP Democratic Reform critic Craig Scott told CBC News on Sunday that "whether or not a higher-up in the Conservative Party knew about the deal is exactly what we all need to know."

According to Scott, Harper could have told Wright to "make this problem go away" before the Deloitte audit was completed, leaving the details up to his chief of staff.

"It's not impossible that that's the scenario," Scott said.

Abolish or reform?

LeBreton's comments about the Senate's ability to survive its current scandal reflect the fresh wind in the sails of Senate abolitionists.

On Friday, Saskatchewan Premier Brad Wall – whose party's official policy calls for an elected Senate – joined the chorus of those suggesting the Senate is too broke to fix and needs to be scrapped outright.

Last week, the NDP renewed its call for the Senate to be scrapped, launching a new website.

In an interview with French-language newspaper La Presse on Saturday, Liberal Leader Justin Trudeau took issue with the NDP's abolition campaign, 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 NDP Leader Thomas Mulcair's campaign.

"We'll have to improve it," he continued, favouring Senate reform.

Either way, it's not right for federal leaders to do this alone, Trudeau said.

"It's a mistake for Harper not to consult the provinces in reforming the Senate. It's a mistake for Thomas Mulcair not to talk to the provinces about his plan to abolish it," Trudeau also told La Presse.

According to Canada's Constitution, changes to the Senate cannot proceed without the approval of seven of 10 provinces representing at least 50 per cent of Canada's population.

RCMP still considering investigation

The RCMP confirmed in a letter to the Senate last week that it is conducting a review of the Deloitte reports on the expenses of three senators: Patrick Brazeau, Mac Harb and Duffy.

All three claimed housing expenses that were deemed inappropriate. Brazeau and Harb were asked to repay tens of thousands of dollars in expenses but have commenced legal challenges to the Senate committee's findings against them .

The Mounties have asked the Senate for policy documents relating to living expenses and travel to determine "whether there are grounds to commence a criminal investigation."

A former RCMP superintendent said Sunday he's never seen the degree of political control over the Mounties that exists now, adding that it "does not bode well" for an objective police investigation of the Senate scandal.

Gary Clement, a 30-year veteran of the force who spent more than half those years working in the national capital region, told CTV Sunday that the thrust of the RCMP investigation will likely centre around Section 122 of the Criminal Code and breach of trust.

"From my read of the act and what's been alleged through the media, I think they've got pretty strong grounds," Clement said.

Duffy used the fact that he had already repaid his questionable expenses as a reason to refuse to co-operate with the Deloitte auditors. His repayment – not known at first to have been Wright's money, not Duffy's – was cited by top Conservative senators as the reason why a Senate committee report deleted language that was critical of Duffy's actions.

Second Duffy review ahead

While Harper will have to face the opposition parties in the Commons this week, Conservative Senator David Tkachuk will have to face the Liberals in the Senate.

Tkachuk, who is the chair of the Senate's committee on internal economy, raised several eyebrows in an interview he gave Maclean's last Thursday, when he said he sought "advice from all kinds of people" including the PMO, his colleagues, fellow senators, members of the media and others before submitting the final report on Duffy's expenses.

CBC News reported last Thursday the Duffy report was altered in the hours before it was tabled, to remove the finding that Duffy's travel pattern does not support his declaration of a primary residence in P.E.I.

Cowan told CBC News the latest revelations by Tkachuk raise more questions about undue influence the prime minister's office may have had on the final Duffy report.

"I'm going to have a series of questions for Senator Tkachuk on Tuesday afternoon when the Senate meets," Cowan said.

The Senate decided to send Duffy's audit report back to its internal economy committee for a second review and while Cowan would have preferred that the Senate refer the matter to the RCMP, Cowan has asked that Tkachuk hold the meetings in public.

As of Sunday, Cowan said he had not heard back from Tkachuk.

In an email to CBC News on Sunday, Tkachuk confirmed he would be back in Parliament this week after being away due to recent surgery.

Other investigations continue

The federal ethics commissioner, Mary Dawson, is examining whether Wright violated the Conflict of Interest Act when he gave Duffy more than $90,000 to repay his housing expenses.

The Senate Ethics Officer Lyse Ricard has also been asked to look into Wright's gift to Duffy.

Deloitte is also currently reviewing the expenses of Senator Pamela Wallin, who has claimed approximately $321,000 in travel expenses since September 2010.

Sources have told CBC News Wallin may have to repay thousands in ineligible travel claims.

The report on Wallin's expenses is expected before the Senate rises in June.  

With files from CBC's Susana Mas, The Canadian Press