MONTREAL—There was a time, not long ago, when the Quebec sovereignty movement swore by the model of the European Union. In the lead-up to the 1995 referendum, it provided the template for the project of a post-secession partnership with the rest of Canada. Back then, most leading sovereigntists sincerely believed in an EU-style Canada-Quebec union based on joint political institutions, a common currency, and open borders. They probably still do. Others endorsed the idea strictly for marketing purposes.