Some observers, namely a few political scientists sympathetic to the NDP, have wondered in the last few days why the Leader of the Liberal Party, Justin Trudeau, decided to raise the issue of national unity at the Maclean's debate last week.While campaigning in Québec on the eve of la Fête nationale du Québec on June 23, NDP Leader Thomas Mulcair stated the following while speaking to the media « It is clear, the Sherbrooke Declaration remains a core proposition of our offering to Quebecers ».The NDP Sherbrooke Declaration states that « the NDP would recognize a majority decision (50% plus 1) of the Quebec people on the event of a referendum on the political status of Quebec ».Some will wonder why this is in fact controversial.It is important to go back to what the Supreme Court stated in 1998 in its landmark decision on Quebec secession that led to the adoption of the Clarity Act by Parliament in 2000.In essence, the Court concluded that Quebec cannot secede unilaterally under Canadian or international law. However, and this is not trivial, the Government of Canada would have to enter into negotiations with the Quebec government if Quebeckers expressed a clear will to secede.The Supreme Court confirmed that this obligation to enter into negotiation on secession can be created only by "a decision of a clear majority of the population of Quebec on a clear question to pursue secession." The Supreme Court mentioned the words “clear majority” no less than 13 times, and also referred to a “strong” majority.The Supreme Court also urged the Governement of Canada not to determine in advance what constitutes a clear majority: “it will be for the political actors to determine what constitutes 'a clear majority on a clear question' in the circumstances under which a future referendum vote may be taken.”This is very wise advice: there is a qualitative dimension to assessing clarity, which begs for a political assessment to be done in full understanding of the actual circumstances.Furthermore, setting any kind of threshold in advance would expose us to the risk of leaving such a serious decision to the results of a possible judicial recount or the examination of rejected ballots. That would put us all in a very difficult – even senseless – position.By stating unequivocally he would repeal the Clarity Act that gave effect to the Supreme Court judgment on Quebec secession, it is Mr. Mulcair, not Mr. Trudeau, who is making the Sherbrooke Declaration an issue of the current election campaign. As someone who aspires to become Prime Minister, he should explain to Canadians why he disagrees with the Supreme Court. And more importantly why we don't ever hear him talk about the Sherbrooke Declaration outside Québec.Robert Asselin is Vice-President, Policy and Research at Canada 2020. In 2014, he was a Visiting Public Policy Scholar at the Woodrow Wilson Center for International Scholars in Washington, D.C. and was Associate Director of the Graduate School of Public and International Affairs at the University of Ottawa from 2007 to 2015. Mr. Asselin is an advisor to the Leader of the Liberal Party of Canada.