Active opposing the Italian colonial war in Libya, Bordiga was active in the Italian Socialist Party founding the Karl Marx Circle in 1912. He rejected a pedagogical approach to political work and developed a theory of the party whereby it constituted a non-immediate from of organisation which included some people not sociologically working class. He was profoundly opposed to representive democracy which he associated with bourgeois electoralism declaring:
“Thus if there is a complete negation of the theory of democratic action it is to be found in socialism.” Il Socialista, 1914
Therefore he opposed the parliamentary fraction of the Socialist Party members of parliament being autonomous from control by the party. In common with most socialists in the Latin countries he campaigned against freemasonry as a firm advocate of secularism.
Following the Russian Revolution of October 1917 he rallied to the Communist movement and formed the Communist Abstentionist fraction within the Socialist Party. Abstentionist in that it opposed participation in bourgeois elections and it was this fraction that would, with the addition of the former L'Ordine nuovo grouping in Turin around Gramsci, that formed the backbone of the Communist Party of Italy (PCd'I) founded at Livorno in January 1921. This was after a long internal struggle in the Italian Socialist party which had voted as early as 1919 to affiliate to the Third International but had refused to purge its reformist wing. In the course of this struggle Bordiga had attended the 2nd Congress in 1920 where he added 2 conditions to the 19 conditions of membership proposed by Lenin.
Nevertheless he was attacked by Lenin in “Left-Wing Communism: An Infantile Disorder”. He became leader of the PCd'I until his arrest in 1921. After succesfully defending himself at his trial, he nevertheless refused to rejoin the Executive Committee and in 1924 he refused to be named as the Vice President of the party. He attended his last meeting of the Executive Committee of the Communist International in 1926. In 1930 he was expelled from the PCd'I for taking the defence of Leon Trotsky.
After 1944 he made anonymous contributions to such journals as Battaglia Comunista, Il Programma Comunista, and Promoteo. He produced a theory of the Soviet Union which presented it as being fundamentally capitalist.
Whilst remaining pro-Lenin, he was a constant critic of Leninism.