Nationalist and authoritarian contestation of global governance is both a recent trend and an old challenge to international institutions. Relying on case studies of the EU and Mercosur (Brexit, Poland, Venezuela), this research will focus on the reversal to authoritarianism and nationalism in member States of regional integration organizations. This research rests on the hypothesis that these organizations, as decentralized agents of International Public Authority, are a litmus test for the analysis of international institutions‘ resilience to nationalist and authoritarian tendencies. It will also contribute to the analysis of identity, security and democracy, as these values are claimed by both the member States and the regional organizations. The nationalist or authoritarian pushback in Member States of regional organizations is a fundamental challenge to international institutions; as such, this research will give an essential perspective on current challenges to Public Law.