Tom Mulcair has promised Quebeckers that under an NDP government a future referendum on independence would be decided by the 50+1 rule. Should he win, it's a promise he'll find hard to keep, for much the same reason that he won't be able to abolish the Senate. There's this pesky thing called the Constitution…A glance at the Supreme Court's 1998 Secession Reference shows Mulcair's problem.First, the Court says that secession negotiations are obligatory only if a “clear majority” of Quebeckers vote yes. While the justices do not say what, exactly, that number is, everyone agrees it is more than 50+1.Second, secession will require a constitutional amendment, which means the provinces must be onside. Indeed, as this will almost certainly involve changes to the amending formulas, all of the provinces will have to be onside.Now imagine a Yes vote of 50.1%, after which Prime Minister Mulcair opens negotiations. Note that the provinces would not be obliged to participate. But if even one or two chose to stay home, there could be no constitutional amendment, so the process would fail. Mulcair consistently ignores this glitch. For example, he maintains that Pierre Trudeau's participation in the 1980 referendum has already set the precedent for 50+1. What he doesn't say is that the BNA Act was still in effect then and that, under that regime, the provinces' role in such matters was far less clear.Since then, the Supreme Court References on Patriation (1981) and Secession (1998), along with the new amending formulas in the 1982 Constitution, have firmly established the provinces as full partners in constitutional change. So even if Trudeau did act alone in 1980, no prime minister could do so today.Now, to be clear, I am NOT saying that 50+1 can't be the threshold for negotiating independence. As Adam Goldenberg notes, while the SCC ruling states that a clear majority obliges Ottawa and the provinces to negotiate, nothing in the ruling prevents them from choosing to do so if the majority falls below this. So they are free to use the 50+1 rule, if they wish. In fact, this is the way Quebec has always tried to frame the secession debate.Sovereigntists like former Quebec Premier Jacques Parizeau argued that the Constitution was irrelevant. Secession, he insisted, is a political question, not a legal one. It occurs outside the constitutional order and governments should respond accordingly.Had the Yes vote reached 50+1 in 1995, Parizeau even planned to issue a Unilateral Declaration of Independence. He apparently hoped this would provoke a constitutional crisis and force the federal government to look beyond the Constitution for a “political” solution.Whether or not Mulcair realizes it, he has aligned himself with Parizeau's approach. By adopting the 50+1 rule, he is in effect rejecting the legal route and opting for the political one. And now he wants all of Canada to join him. Should we?Quebec separation would be a difficult and divisive undertaking. For example, there would be serious issues around the division of assets and sharing the national debt. Views of what is fair could diverge profoundly.Questions over the borders would be even more volatile. For example, the Cree Nation held its own referendum in 1995 and voted 96% in favour of remaining with Canada. Quebec, on the other hand, has always insisted that the borders are non-negotiable. Given that Ottawa has constitutional responsibility for Aboriginal peoples, how would it respond to such a demand from the Cree?We're only scratching the surface here, but it's enough to see why secession negotiations would be tense and acrimonious, not just with Quebec, but within the rest of Canada (ROC). In these circumstances, the prime minister's first responsibility would be to ensure the unity of the ROC, while seeking a fair exit agreement with Quebec.Opting to work from within the Constitution is the logical first choice. It would ground Ottawa's approach in authoritative rules and ensure the legitimacy of its leadership role. It also calls on Ottawa to involve the provinces, which brings us to the question of “clarity.”In the MacLean's Leadership Debate, Mulcair attacked Justin Trudeau for not having a firm number on the clarity question. Actually, this puts the cart before the horse. As Trudeau noted in a press conference on Sunday, the time to decide this is after a referendum call, not before. We need to assess the context before making a decision.I would add that this decision should be made in consultation with the provinces, not by Ottawa alone. The sensible plan is this: if and when another referendum is called, the PM should convene the nine premiers; together they should assess the situation; and only then should the PM seek agreement on a definition of clarity.Whether the ROC would opt for the 50+1 rule or some higher number remains to be seen, but at least it would be united. Should it set the bar too high for Quebec's liking, that government might well declare independence, throwing the ROC into a constitutional crisis. Ottawa's only option then would be to improvise, that is, to engage in a “political” process. No one knows where this would lead.So in the end the ROC might well be forced into a political process, but given the volatility of the issues and the acrimonious history around such ruptures, it makes little sense to start there. I get why Quebec nationalists would prefer this but, from the viewpoint of the ROC, it effectively removes the constitutional foundation we would need to help ensure that Canada survives the traumatic event of Quebec's departure. We can always move from the legal to the political, if required. But we can't do the reverse, so why start there?Finally, let me note that a worrying pattern seems to be emerging in Mulcair's campaign. Whether it is the secession of Quebec, abolition of the Senate, or launching a national childcare program, he has shown a tendency to treat the Constitution and the provinces as an afterthought. It's curious for a politician who spent his formative years in a provincial legislature—Quebec's National Assembly.Then again, maybe that explains a lot.Dr. Don Lenihan is Senior Associate, Policy and Engagement, at Canada 2020, Canada's leading, independent progressive think-tank. Don is an internationally recognized expert on democracy and Open Government. His recent projects include chairing an expert group on citizen engagement for the UN and the OECD; and chairing the Ontario Open Government Engagement Team. The views expressed here are those of the columnist alone. Don can be reached at: [email protected] or follow him on Twitter at: @DonLenihan