Abstract. This article analyzes party responses to European integration in Finland andSweden.We argue that such responses are shaped by seven explanatory factors: basic ideology, public opinion, factionalism, leadership influence, party competition, transnational links, and the development of integration. Each factor can lead to a positive or a negative evaluation of the European Union. In the empirical analysis, the sample includes all parties represented in the respective national parliaments, and the research material consists of party documents, parliamentary votes, statements by leading party figures, public opinion surveys, direct observation and interviews. Party competition and leadership influence are the strongest factors in the Finnish case, while public opinion and factionalism are the strongest factors in Sweden. Issue avoidance combined with the secondary importance of the EU in party politics explain why parties have been relatively successful in containing internal factionalism and discord, especially in Finland.