The governor general is being asked to weigh in on the controversy over Senate residency requirements and whether the current appointment process meets constitutional obligations.

"The recent Senate scandal has resulted in a growing crisis of confidence in the upper chamber of government," writes NDP MP Charlie Angus, in a letter addressed to Gov. Gen. David Johnston. "If the process for choosing senators cannot be shown to be in line with constitutional obligations, then Canadians' trust in both levels of government could be eroded further."

The Constitution states that a senator must own $4,000 worth of property in the province he or she represents. As well, senators "shall be resident in the province" for which they are appointed. But, as Angus notes in his letter, questions have been raised whether a number of senators, including suspended Senator Mike Duffy, meet that requirement.

NDP ethics critic Charlie Angus says Prime Minister Stephen Harper insists the senators he appointed fulfil Senate residency requirements without being clear about what those requirements mean. (Sean Kilpatrick/The Canadian Press) Angus, the ethics critic for his party, writes that the prime minister has repeatedly insisted that the requirements to be a senator have been clear for 150 years and that all senators meet those requirements, "Yet we have been unable to learn from the prime minister's office exactly what those requirements are."

Angus poses five questions to Johnston, who appoints all senators based on the prime minister's recommendations, to "outline the process from your office's view."

He asks:

What is the 150-year-old definition of constitutional residency?

Has Johnston's office asked for legal advice on the residency question?

What is the process for vetting potential and appointed senators?

What role does Johnston's office play in ensuring nominated senators are constitutionally eligible?

What measures have been put in place to ensure nominees respect constitutional requirements?

In 2013, then-government Senate leader Marjory LeBreton, citing a legal opinion, said all that was required was for a senator to declare they were resident in the province.

"Senators sign a declaration of qualification," LeBreton told reporters at the time.

"That's the document that secures your legitimacy to sit in the Senate."

But the Constitution also says that only the Senate has the power to consider challenges to senators' eligibility to sit in the Senate.