This article argues that citizens employ proxies rooted in attitudes about domestic politics when responding to survey questions about the European integration process. It develops a model of public opinion toward European integration based on attitudes toward the political system, the incumbent government, and establishment parties. With the help of data from Eurobarometer 34.0, the study tests political and economic models of public support for membership in the European Union in Belgium, Denmark, France, Germany, Ireland, Italy, and Portugal. The analyses show that system and establishment party support are the most powerful determinants of support for membership in the European Union. The results also suggest that the relationship between economic factors and support previously reported in research on public opinion toward European integration is likely to be mediated by domestic political attitudes.