ABSTRACT In 2021, the European Union (EU) started to use material sanctions to punish democratic backsliding in Hungary and Poland. This policy change presents a puzzle for the existing literatures on international responses to backsliding. We theorise two distinctive processes that can account for why EU policy changed from inaction to enforcement. First, once the issue of backsliding in a member state has attained public salience across the other member states, their mainstream parties face domestic electoral incentives to support sanctions against illiberal governments abroad. Second, once backsliding governments also disrupt intergovernmental policy cooperation and threaten common policies at the EU level, even those actors who had been reluctant to defend EU values become more inclined to use sanctions. We demonstrate the plausibility of our explanation with evidence, first, of the increasing public and electoral salience of backsliding in other EU member states, and second, of the occurrence of a negative intergovernmental spillover through increasing attacks by backsliding member state governments against common policies.