What explains when Members of the European Parliament (MEPs) defect from their EP party group? While previous research has focused on the policy distance between an MEP's national party and her party group, it has been overlooked that not all issues are equally important to national parties. As parties prioritize certain issues over others, we argue that it is both the distance and the salience of the issue for the MEP’s national party that explains defection. To test our theoretical claim, we explore more than 400,000 vote decisions across 1,948 different roll-call votes in four issue areas – agriculture, environment, social policy and external trade – from 1979 until 1999. To measure policy salience and distance, we combine roll-call analysis with data from the Euromanifestos Project. Our findings have important implications for understanding the dynamics of party competition and the role that national parties play in the European Parliament.