Through a comparative analysis of eight European referendums in Spain, France, and Ireland since the Maastricht Treaty of 1992, this paper considers the effects of the referendum context on the dynamics of competition among opposition parties. By generating a "yes" cartel, referendums displace the dominant cleavage between government and opposition in majority democracies, thus increasing factionalism and reducing the capacity for mobilization of mainstream opposition parties, which are torn between ideological support for European integration and strategic opposition to the government, which reinforces the role played by peripheral opposition parties.