The rise of populist radical right parties (PRRPs) in a growing number of European Union (EU) member states as well as inside the European Parliament (EP) has triggered concern over their ability to drive further contestation of European integration. Using EU enlargement as a test case, we adopt a discourse-based approach to assess the impact of PRRPs’ collective strengthening inside the EP. Our empirical analysis applies political claim analysis to an original dataset of over 6’000 hand-coded statements about membership from the past three EP mandates to trace the emergence of an increasingly coherent, oppositional discourse by PRRPs towards a further widening of the EU. Moreover, we show that PRRPs contribute to a generalised hardening of positions towards aspiring member states, but are not able to impose their identity-based framing upon other parliamentary actors. Instead, we suggest that mainstream party groups accommodate the PRRPs’ essentialist discourse by shifting from technical, conditionality-based reasoning towards more political arguments articulated around human rights, bilateral issues and institutional efficiency. Our findings feed into debates on the transnational cooperation of PRRPs and the political impact of Euroscepticism.