This study provides new insights regarding how the EU dimension relates to the left-right dimension in national politics. It does so by examining to what extent parties’ oppositional behaviour in EU affairs in parliament is a function of their distance to the government, other parties, the voters and party supporters on the EU dimension and the left-right dimension, respectively. The results suggest that oppositional behaviour is determined by two parallel dynamics: When expressing opposition, political parties are receptive both to the positions of the government and other parties on the EU dimension, and to the positions of voters and supporters on the left-right dimension. Our results thus indicate that the politicization of EU affairs in national parliaments primarily is an elite phenomenon. In their relation to the voters and supporters, parties still primarily navigate along the traditional left-right dimension.