Effective enforcement is essential for international cooperation. Accordingly, the European Union (EU) has developed a number of formal tools for enforcing its laws and policies in the members states. The application of these tools, however, has been slow, timid and uneven. One of the possible reasons for this is the fear that enforcement by supranational institutions (the Commission and the Court of Justice of the EU) will provoke public backlash in the member states targeted by sanctions. Systematic empirical evidence for such effects of enforcement is, however, lacking. This paper develops hypotheses about the factors affecting the perceived legitimacy of enforcement actions. The hypotheses capture the possible influence of procedural fairness, descriptive norm prevalence, and the likely effects of the sanctions on the future of the EU. These hypotheses are tested with a survey experimental design administered to a nationally-representative sample (N=1,200) in Poland. The context of the study is the enforcement of rule of law, and the focus is on the substantial financial sanctions imposed by the EU with regard to judicial independence in particular. The results show that the perceived legitimacy of the enforcement actions is strongly related to exclusive national identity, the perceived importance of the rule of law as a norm, and partisan affiliations. We find some evidence that providing information about the prevalence of public support for judicial independence in the country increases significantly the perceived legitimacy of enforcement actions. No evidence is found about possible effects of information about Polexit, future deterrence effects of the sanctions or their procedural fairness or unfairness. Altogether, these results suggest that the perceived legitimacy of enforcement actions is very hard to be influenced when the issue at stake is relatively salient and politicized.