Power versus Ideology: Political Group Switching in the European Parliament

Why do legislators switch party? We seek to identify whether "party switching" is mainly determined by power (to join a more influential party) or ideology (to join a party with closer policy goals). We focus on the 557 cases of political group switching in the European Parliament between 1979 and 2014. We find that most of these cases were from smaller, more marginal, and oppositional groups, to larger, more pivotal, and governing groups. Nevertheless, we also find that ideological congruence (between an MEP and his or her prospective group) was an important determinant of political group switching.