Voter-endorsed challenges to international institutions present a growing problem for international cooperation. This talk discusses the challenges that unilateral, voter-endorsed attempts by one member state to withdraw from or renegotiate the terms of existing international institutions create for the institution’s other member states. It argues that such voter-endorsed “cooperation challenges” tend to reverberate abroad: First, they influence public and elite views about the merits of the international institution and thus can create contagion effects, but can also have stabilizing effects depending on how the they highlight the risks and opportunities associated with exit and renegotiation. Second, the loss of, or change in the distribution of, cooperation gains can have considerable effects on the institutions’ other member states. Finally, these reverberations abroad influence how member states respond to unilateral voter-endorsed “cooperation challenges.” Drawing on comparative case studies of a diverse set of voter-endorsed challenges to international institutions and on survey evidence from the EU-27 as well as a panel of Swiss respondents, the talk shows that this framework can help us better understand the (non-)existence of contagion effects across member states, variation in the responses of the remaining member states to voter-endorsed challenges to international institutions, and the ultimate outcomes of such cooperation challenges.