How do challenger parties establish themselves in Western European party systems? In this dissertation, I analyse in four empirical contributions how Green and radical right parties change programmatically and how they extend their support base. The first chapter traces the evolution of challenger parties in Western Europe, reviews existing explanatory approaches and proposes issue and social group yield theory as the theoretical foundations of this thesis. Chapter two examines the role of intra-party cohesion in issue diversification. Using a multilingual transformer model to measure intra-party cohesion, the chapter finds that radical right and Green parties are the most cohesive parties on their core issues. However, while cohesion leads to more salience and clarity on core issues, it is less important for peripheral issues on which party supporters prefer ambiguity. My third chapter scrutinises how the socio-economic context shapes the framing of migration by the radical right. Analysing social media posts, classified by large language models, demonstrates that the radical right primarily relies on nationalist and identity-based frames of migration, particularly in districts with economic or cultural grievances. Moreover, once the radical right poses an electoral threat, the centre-right adopts similar cultural frames, whereas the centre-left frames migration more positively. My fourth chapter probes whether appointing group spokespersons increases persuasion in political justifications. The results from a 2×2 survey experiment indicate that group spokespersons are as persuasive as other politicians. Yet, they positively affect how group sympathisers think of a policy advancing the group’s interests. By contrast, they reduce policy support for salient issues. The fifth chapter scrutinises whether the radical right’s gender appeals contribute to the gender gap in far-right support. Combining panel data with data on the party branches’ gender appeals, the evidence shows a heavy reliance of the radical right on conservative gender appeals. These have a mobilising effect for male but not for female voters.
Mirko Wegemann is a PhD researcher at the European University Institute. He is a political scientist primarily interested in political behaviour and party competition. In his dissertation, he analyses how challenger parties establish themselves in Western European party systems. He analyses their political rhetoric and its effects on the electorate using computational text analyses and experimental methods. Before coming to the EUI, he received his bachelor's and master's degree in Social Sciences from the Humboldt-University of Berlin.