International organizations promoting democratic governance and human rights are increasingly challenged by some of their own member states. To better understand this dynamic, we propose a distinction between the illiberal ideology of political parties and their regime environment, aiming to examine the international behavior of actors extending beyond autocratic governments. We argue that the domestic regime environment plays a pivotal role in influencing the extent to which illiberal parties engage in contestation to undermine liberal norms on the international stage. We expect contestation behavior to be primarily driven by illiberal parties seeking to diminish the influence of liberal international politics on domestic power structures. Moreover, we contend that government participation acts moderates illiberal parties’ contestation behavior. To test our expectations empirically, we study roll call votes in the Parliamentary Assembly of the Council of Europe (PACE), one of the most powerful international parliaments promoting liberal values. Drawing on an original dataset that records approximately 500,000 individual votes cast in PACE decisions, we find evidence for substantive contestation by illiberal parties, especially those representing illiberal regimes. Only illiberal governments in liberal systems moderate themselves at the amendment stage. Our study has implications for the potential threat that emerging illiberal actors pose to international liberal institutions.