This article is an analysis of the relationship between the New Democratic Party of Canada and Québécois nationalist ideas during the time period immediately preceding the 2015 Canadian federal election, when the NDP’s popularity in Québec was at a very high point. Using surveys of federal NDP members, potential voters, and Members of Parliament, it quantitatively examines opinions about Québécois nationalism and explores the factors that influence these opinions. It illustrates that, in English Canada, there is a gap between principles and policy in the attitudes of federal NDP members, potential voters, and Members of Parliament when it comes to Québécois nationalism. While these English Canadian NDPers generally accept the principles that the Québécois are a nation and that the federal government should treat Quebec differently when necessary, they are opposed to the policy of allowing the Quebec provincial government to opt out of federal programs with financial compensation. For their part, federal NDP members, potential voters, and Members of Parliament in Quebec are much better at aligning their principles with their policy positions. The conclusion of the article explores the implications of these results for debates within the NDP concerning Canadian federalism and the place of Quebec in Canada.