The top court will advise the federal government what it will take to reform or abolish the upper house.
The court announced Thursday it will hand down its opinion on April 25.
If the court sets a high constitutional bar that must be scaled to alter the Senate, it may guarantee the maligned chamber is never changed.
If it sets a lower bar, the Senate could eventually be overhauled or eliminated altogether.
Prime Minister Stephen Harper sought the top court's advice after a number of provinces strenuously objected to his attempts to unilaterally proceed with some relatively modest reforms to the upper house.
During a three-day hearing last November, the federal government argued it doesn't need provincial consent to create a non-binding consultative election process for choosing senators or to impose term limits on them.
And federal lawyers urged the eight justices who heard the case not to set the bar so high that Senate reform would prove to be impossible.
But most provinces countered that such reforms would alter the fundamental character of the Senate and, thus, require a constitutional amendment approved by at least seven provinces representing 50 per cent of the population (the 7-50 formula).
They argued that the Senate was an essential part of bargain struck at Confederation, where an unelected, independent chamber of sober second thought with equal regional representation was created to counterbalance the dominance of populous provinces in the elected House of Commons.
Consequently, most provinces maintained the Senate can not be changed without substantial provincial consent.
Harper, who has threatened to abolish the Senate if he can't reform it, has also asked the court whether abolition would require the approval of seven provinces or all 10.
The federal government maintains abolition would require the 7-50 amending formula. Most provinces maintain it would require unanimity.
The court's opinion could scupper Harper's reform proposals, which he had hoped could be achieved without reopening the constitutional can of worms. He has said he has no interest in plunging the country into another round of wrenching — and ultimately fruitless — federal-provincial wrangling, as occurred in the 1990s with the Meech Lake and Charlottetown constitutional accords.
Confining negotiations to the Senate would likely prove impossible. Quebec's new premier, Philippe Couillard, has vowed he won't negotiate Senate reform without putting Quebec's traditional demands — for recognition of the province's distinctiveness, among other things — back on the table.
NDP Leader Tom Mulcair is championing Senate abolition and has said he's willing to risk constitutional negotiations to make it happen. But his goal will be all the harder to attain if the top court decides it would require unanimous provincial approval.
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