Daniel Jutras and John Hunter, appointed as "amici curiae" or friends of the court, agree that abolishing the upper chamber would require the unanimous approval of Parliament and all provinces.
And they agree that imposing term limits on senators would require the consent of at least seven provinces comprising 50 per cent of the population — the so-called 7/50 amending formula.
But in a joint factum filed with the court, the two disagree on whether the federal government can proceed unilaterally to set up a mechanism to elect senators.
One amicus, not identified in the factum, maintains that as long as the elections were strictly non-binding public consultations, the prime minister's constitutional discretion to appoint whomever he chooses would remain unfettered. Hence, no constitutional amendment would be required.
"The holding of a non-binding consultative election is more akin to a referendum than a direct election," the amicus argues, adding that the prime minister remains "at liberty to recommend anyone (for appointment to the Senate), having consulted anyone or no one."
That said, the amicus says the two Senate election bills proposed by Prime Minister Stephen Harper's government thus far would not necessarily "pass constitutional muster" because they went too far in requiring the prime minister to appoint only the winners of Senate elections.
The other amicus argues that elections, binding or not, would fundamentally change the Senate's constitutional status as an unelected, independent chamber of sober second thought. Hence, elections could only be introduced with the approval of seven provinces representing 50 per cent of the population.
"On this analysis, it is irrelevant whether the consultative electoral schemes contemplated ... are binding or non-binding, good or bad, constraining of constitutional conventions or otherwise," the factum says.
"The key is whether the proposed enactments modify the entrenched dimensions of the Constitution of Canada."
Prime Minister Stephen Harper's government has asked the country's top court for advice on the constitutionality of its plans to reform the Senate — or abolish it, should reform prove impossible.
The amici are the last to file factums in the case, which is to be heard in November.
In its factum, the federal government argues that it can unilaterally impose term limits and create a process for electing senators. Abolition, it maintains, would require 7/50 approval.
All ten provinces and two territories have also filed factums and most disagree with Ottawa's arguments.
Only Ontario and Saskatchewan agree the federal government can unilaterally impose term limits, provided they are long enough (at least nine and 10 years respectively). The rest maintain 7/50 approval would be needed.
Only Alberta and Saskatchewan agree the federal government can unilaterally create a consultative election process for senators. The rest maintain at least seven provinces would have to consent.
Only Saskatchewan and Alberta agree abolition of the chamber could be achieved through the 7/50 formula. The rest argue that unanimous consent would be required.
However, all provinces, except Quebec, agree with the federal argument that it can unilaterally eliminate the constitutional requirement that a senator must own $4,000 worth of property in the province he or she was appointed to represent.
The amici side with Quebec in arguing that such a change would require Quebec's consent because it is the only province in which senators are actually appointed to represent specific electoral districts.
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