This thesis deals with the ways distinct types of parties in government or in opposition have adapted their strategies of competition to handle EU integration. The thesis draws on data from four countries – Britain, France, Ireland and Spain – over the past 15 years of electoral politics. The thesis provides a comparative and longitudinal analysis of their strategies of politicisation and of depoliticisation in relation with the parties? different statuses within their party systems (incumbent, mainstream opposition and peripheral opposition) and in three distinct arenas (national elections, European elections and EU referendums). It investigates how these parties have handled EU issues in three complementary dimensions of their activities: in their strategies of political communication, in intra-party debates over the EU and in their responses to the consolidation of the European electoral arena. The thesis uses a mix-method approach based on claims-making analysis, thick qualitative comparisons and descriptive statistics. Contradicting the idea of a gradual europeanisation, it first provides evidence for the stable marginalisation and nationalisation of EU affairs. Second, while the rise of euroscepticism is often discussed, the thesis demonstrates that conflicts over EU matters are rarely prominent in practice, and that they remain sporadic and confined to certain electoral channels, with mainstream parties actively compartmentalising the potential divisive effects of EU issues within their organisations. Third, it points to the centrality of the twofold behaviour of opposition parties, explaining why the politicisation of EU matters has remained contextual and less linked to the role of radical parties than is usually recognised. It finally delineates a process of increasing seclusion between the national and European political arenas that can be observed through the trajectories of MEPs careers in relation to their domestic political spaces or through the disengagement of national parties from campaigning in EU elections.