In recent years, populist radical right parties (PRRPs) have continued to establish themselves in parliaments across Europe. However, there is little research on party responses in parliaments. This article explores how mainstream parties have dealt with the Alternative for Germany (AfD) in state parliaments. Its contribution is twofold: theoretically, it links the existing literature on party responses to the parliamentary arena and proposes a comprehensive framework for analyzing party responses in parliament, distinguishing between the formal and the policy level. Moreover, it tries to understand the variation of responses by emphasizing three important factors: party ideology, the government–opposition divide, and the federal structure of parties. Empirically, the article explores the crucial variation of response patterns toward the AfD at the subnational level, which is often neglected in the study of PRRPs. The results show that party responses reflect an ongoing learning process with no ‘magic formula’ in sight. Overall, the article underlines the importance of party responses in the initial phase for the PRRPs’ impact and offers substantial theoretical and empirical impetus for future research.