An increasing number of member countries of the European Union (EU) are heading for ratification through referendum of the yet to be adopted Constitution for Europe. Such popular votes will add an additional ratification constraint and empirical models will require information on the posi- tions of the voters on various issues related to the new Treaty. This information can come only from mass surveys. In this paper we discuss the ways in which data from mass surveys may be used to infer the policy positions of voters on the most salient aspects of the constitutional Treaty. We also propose a way in which these policy positions may be connected with information on the policy positions of the member states' governments. Combining this information, which is almost imperative for systematic tests of two-level games, allows us to demonstrate that certain referendums are likely to lead to ‘involuntary defections'. In addition, our analysis shows that the prospect of referendums has already led to changes in the draft Constitution.