As a Marxist, Rappoport campaigned in the French Section of the Workers' International ("Section Française de l'Internationale Ouvrière", SFIO). In 1914, he denounced the SFIO's acceptance of the First World War. An early Zimmerwaldian. he was neither a reformist nor a modernist in his ideology. With Paul Vaillant-Couturier and Albert Treint, Rappoport was a representative of the Comintern ("Third International"; 1919–1943). At the SFIO's Tours Congress in December 1920, he was part of the majority who founded the French Communist Party ("Parti communiste français", PCF), and was elected to the Steering Committee. Although in 1922-1923 he supported the Frossard centre of the group, following the Stalinization of the PCF in the late 1930s, he strongly disagreed with the party line and support for the Soviet Union, and he left the PCF in 1938.