During his adolescence he joined the Italian Socialist Party, of which he became suddenly a national leader at the head of the revolutionary wing, known as Massimalisti; other notable members of this far-left wing were Costantino Lazzari and Benito Mussolini. In 1919 Bombacci became the Secretary of the PSI and in the general election in the same year he led the socialists to their best result ever; the PSI won the election with 32.3% and became the first party by votes and seats. Then the following year, Bombacci resigned as Secretary of the Socialist Party, due to some tensions with the right-wing of the party led by Filippo Turati.