We design a novel experiment to explore the multidimensional foundation of partisanship in Europe. Partisanship is considered as a key mechanism of affective polarization with increasing hostility and distrust across party lines between in- and outgroups. Combining a conjoint experiment with decision-making games from behavioral economics we examine the cleavage structure of in- and outgroup behavior in a broad cross-national context of 25 European countries. In these countries, a new conflict around European integration is complementing the traditional left versus right conflict in party competition. We find a dominance of partisanship in relation to other traditional cleavages such as social class and religion, which intensifies affective polarization not only on the classical left right but also the new European integration dimension. We also find surprising importance of EU attachment what we call European identity, which goes beyond nationality. While in-group favoritism exerts a bigger influence than out-group derogation for partisanship, things are the other way around for EU attachment, with stronger effect of out-group derogation.