Ontario is set to argue that the federal government needs the consensus of all regions of the country if it wants to abolish the Senate.
A provincial government source says the province will tell the Supreme Court of Canada that the consent of seven provinces, representing at least half of the population, is not enough to scrap the upper chamber.
The federal government argues that the so-called 7/50 rule is all that is needed to abolish the Senate.
Ontario will say that rule applies only if the federal government wants to set term limits for senators that are less than nine years, or to hold Senate elections. The province will take no position on whether senators ought to own at least $4,000 worth of land in the province they represent.
Newfoundland and Labrador and Nunavut, which filed their submissions Thursday, take similar positions.
Ottawa is asking the court for guidance on what it would take to reform the upper chamber and whether it can abolish the body without provincial consultation.
Ottawa says it plans to act
In its submission, the federal government argues "it is constitutionally permissible for Parliament to impose term limits, provide for public consultative processes on Senate appointments, and remove the archaic requirement that a senator" own at least $4,000 worth of land.
The federal Conservatives say they plan to move ahead with Senate reform — or even outright abolition — once the Supreme Court provides guidance over how they may do so.
The New Democrats, meanwhile, are calling for Senate abolition.
Newfoundland and Labrador, Nunavut and Manitoba have all filed submissions arguing Ottawa cannot act alone to set term limits, hold elections for senators or abolish the upper chamber outright.
Manitoba Attorney General Andrew Swan says the upper chamber is basically flawed and should be abolished, but he cautions that the federal government can't go it alone.
The province's submission argues that Parliament does not have the constitutional authority to enact significant unilateral changes to the structure of the Senate or to the selection of its members.
Swan says such changes require consultation and agreement with the provinces.
The rest of the provinces have until today to file their submissions to the Supreme Court.
As the court ponders the government's questions, a scandal over Senate expenses has cast the upper chamber in an unflattering light.
The RCMP is looking into the questionable expenses of former Conservative senators Mike Duffy, Pamela Wallin and Patrick Brazeau, as well as former Liberal Mac Harb, who has since resigned.