Private members’ bills, particularly those from members of the Bloc Québécois, rarely make their way through the parliamentary process. But after passing the House of Commons with strong support from members of all parties, a bill from Yves Perron, who speaks for the Bloc on farming, handily passed a second vote in the unelected Senate on Tuesday.

And perhaps even more surprising, it deals with a contentious issue: Canada’s supply management system, which controls production and sets minimum prices for dairy and poultry products as well as eggs.

Many free-market economists and politicians cast supply management as a legalized price cartel that increases Canadians’ grocery bills. And in negotiations for every one of Canada’s major trade agreements in recent decades, the supply management system has emerged as one of the final sticking points.

[Read from 2016: Safe for Now, Canadian Dairy Farmers Fret Over E.U. Trade Deal]

If Mr. Perron’s bill makes it past the few remaining legislative hurdles and becomes law, it will bar Canada’s trade negotiators from offering any changes to supply management during future trade talks.

Under the system, to avoid price-killing oversupply, farmers are assigned a production quota — effectively a license to produce milk, chicken, turkey or eggs — that they cannot exceed. Until recently, imports were effectively banned through eye-wateringly high import duties.