Résumé : This thesis aims to investigate and reconstruct the processes of transformation of the political culture of the Italian Communist Party in relation to two very important historical events: the crisis of the Welfare State and post-war social compromises and the relaunch of the process of European integration imagined and promoted by Jacques Delors during the 1980s. Starting from the reconstruction of the debate that arose around the causes of the Welfare crisis, the research proposes first of all to verify the influence of the analytical categories that emerged from this debate on the parties of the European left, and in particular the Italian ones. Secondly, the 'Europeanist turn' matured by various socialist parties and by the PCI in the early 1980s is analysed, reconstructing the dialectic established between the debate of these parties and the guidelines of the political action led by Delors, who arrived at the head of the European Commission in 1985. Through the reconstruction of the evolution that characterised the PCI and the nascent PDS's judgement towards the social project implicit in the 'Delors plan' and in the model of integration pursued by him, it is possible to grasp some of the fundamental traits of the ideological and identity transformation that the PCI would undergo between the second half of the 1980s and the beginning of the 1990s