What explains the change and stability we have seen recently across European party systems? In Political Entrepreneurs: The Rise of Challenger Parties in Europe, Catherine E. De Vries and Sara B. Hobolt offer a theory that explains both the resilience of dominant parties and the success of challenger parties. They define dominant parties as those that have been in the national government; challenger parties, in contrast, are those that have never held national office. Once a challenger party is part of the national government, it ceases to be a challenger party. To explore the dynamic relationship between dominant and challenger parties, De Vries and Hobolt use both rich case studies and cross-national statistical analyses.

Their theory comprises four key elements: (1) the political market is an oligopoly with imperfect competition as rules favor dominant parties and voters are more attached to these parties; (2) there is, however, a weakening of this oligopoly that benefits challenger parties as voters become more critical of, and less loyal to, dominant parties; (3) challenger parties act as political entrepreneurs by mobilizing voters around issues that can drive a wedge between the coalitions of dominant parties, namely, European integration, immigration, and the environment and using antiesta