This research project addresses the effects of Europeanintegration on the actualand perceived ideological positions of parties and links the extent ofmainstream parties' ideological convergence to the success and failure ofchallenger parties. Usingmanifesto data from CMP and survey data from CSES, I conducted a series of empiricalanalyses to test these relationships. I obtained important empirical findingswhich have implications for the literature on European integration and partycompetition. My results suggest that more European integration reduces theideological distance among mainstream parties and is associated with morewidespread perceptions of mainstream party convergence among voters. Analysesof the electoral performance of challenger parties point to the effects of bothactual and perceived ideological convergence: challenger parties receive more voteswhen the moderate mainstream parties converge. Likewise, voters who see themainstream parties as ideologically similar are more likely to have voted for achallenger party.