This paper explains how strong resistance to Europe came to be crystallized in the French Socialist Party (PS) during the 2005 referendum campaign on the European Constitutional Treaty. From a historical and institutional perspective, it first shows that from the start of the integration project, resistance to European integration was to a degree part of a latent doctrinal identity of the party, which depended heavily of the balance of power among factions. The article then analyzes how other players on the left put pressure on the PS at the level of cognitive references and the dynamics of mobilization.