As the rules stand, Mac Harb's retirement from the Senate this week ensured his lucrative government pension will last as long as he does – even if an RCMP investigation into his alleged spending violations leads to serious charges, a conviction and a prison term.

Senators who are convicted of an indictable offence can be ejected from the Senate and lose their parliamentary pensions. But if they resign before that happens, they are entitled to all of the benefits they have accrued.

The loophole in the rules for the Senate has allowed former Liberal senator Raymond Lavigne to collect an estimated $79,000 annually even as he sits in a cell at the Ottawa-Carleton detention centre serving six months for fraud and breach of trust. Mr. Lavigne quit the Senate immediately after he was convicted in 2011, avoiding the eviction from the Red Chamber that would have cost him his government-funded retirement plan.

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Mr. Harb, whose office said on Tuesday he could not be contacted, has repaid a total of $231,649 that the Senate says he improperly claimed in living expenses. But he insists he has done nothing wrong and will be vindicated by an Auditor-General's investigation. He retired, he said, because the dispute made it impossible for him to continue work as a senator.

The New Democrats say he was hedging his bets by bowing out of public life. "He's using the Raymond Lavigne playbook," said Charlie Angus, the NDP ethics critic.

The Canadian Taxpayers Federation estimates Mr. Harb will receive $122,989 a year in his retirement from his service as an MP and a senator – amounting to more than $5-million if he lives to 90.

But a proposed change to the rules could mean that, unlike Mr. Lavigne, Mr. Harb and three other senators who are being scrutinized by the Mounties will not be able to rely on a resignation to keep the pension cash flowing if they are found to have broken the law.

John Williamson, a Conservative backbencher from New Brunswick, has introduced a private member's bill that would deny the benefits to any senator or MP who is convicted of a crime that could result in a penalty of more than two years.

While private member's bills do not often get far in Parliament, this one seems likely to have the support of the Conservative government. And it is retroactive to June 3 of this year, which means, if it is passed into law, it would apply to any politician who is found guilty after that date – even if he or she has resigned or retired in the interim.

"The intent of my bill is to remove pensions from lawmakers that break the law," said Mr. Williamson, a former head of the Taxpayers Federation. "From my point of view, and I think from the point of view of a lot of taxpaying Canadians out there, it shouldn't matter whether the senator is still in the Senate or has ducked out early, or even resigned and then the malfeasance was discovered many years later."

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Such a bill could face significant legal challenges given that politicians contribute to their pension plans to ensure that the benefits will be there when they retire. Mr. Williamson says his legislation would allow the contributions to be refunded in case of serious criminality by a politician, but the significant portion of the plan that is publicly funded would be withheld.

In addition to Mr. Harb, a former Liberal, Independent senators Patrick Brazeau, Mike Duffy, and Pamela Wallin (appointed as Conservatives) are under RCMP investigation after audits by the Senate. In all four cases, the Senate asked for certain expenses to be repaid.

But "nobody has ever been kicked out of the Senate despite some pretty wild behaviour," Mr. Angus said. "If the Senate has found that Mac Harb ripped off the Canadian taxpayer, same with Patrick Brazeau, same with Mike Duffy, same with Pamela Wallin, why do they get to go back to work? Why doesn't the Senate take steps immediately to throw them out?"