Who Lets Them In? Party System Structure, Polarization and Legal Constraints on the Parliamentary Breakthrough of the Extreme Right in Southern Europe

What explains the presence (or persistent exclusion) of extreme-right parties (ERPs) and radical-right parties (RRPs) in the national parliaments of Southern European Union member states? While existing literature has extensively examined the electoral behavior of radical-right actors and their voters, far less attention has been devoted to the systemic and institutional conditions that facilitate or constrain ERPs success. This article investigates the impact of party system characteristics, ideological polarization and democratic defense mechanisms on ERPs parliamentary entry across all Southern European EU member states from 1998 to 2025. Drawing on original data and a novel application of Golosov’s Relative Size Triangle (RST), the analysis shows that while ERPs are more likely to succeed in multiparty systems, party system format alone is insufficient to account for their parliamentary breakthrough. Instead, high levels of political polarization and the absence of militant democratic provisions emerge as the strongest predictors of ERP presence in national legislatures. By combining spatial models of party systems with legal-institutional indicators, this study highlights how electoral engineering and constitutional design remain crucial (yet often overlooked) tools for democratic resilience in an era of increasing ideological extremism.