The paper deals with the reactions of the Italian Communist Party to the celebration and the initial reception of the Second Vatican Council. Based upon examination of the Communist press and the documentation of the Communist leadership, it investigates how the Vatican II resolutions modified the Party's electoral and political strategies. Particularly, the research focuses on the political debate concerning the crisis of the Christian Democracy Party after Vatican II and the Communist plan to take advantage of the Catholic protest against the alliance between the Church and political power. In the Communist view, the most important result of Vatican II was the «End of the Constantinian Era». The operation's failure to involve some leaders of the Catholic «dissent» in the electoral rolls for the Senate was to represent one of the proofs of the Communists' difficulty in understanding the depth of this transformation and in reacting to the Catholic crisis of 1968.