He was the author of the order that expelled the pro-National Syndicalist and future Fascist ruler Benito Mussolini from the party in 1914, and, in contrast to Mussolini, was among the foremost opponents in first line to protest against the Italian intervention in World War I.
He was a member of the executive committee of the new grouping, together with Bordiga, Ruggero Grieco, Luigi Repossi and Umberto Terracini, with responsibility for the illegal activities against the increasing success of Fascist forces in repressing left-wing forces.
He moved to Milan, but here he met difficulties to find a regular job, which were aggravated by his expulsion from the PCI in 1929.
Despite the PCI's campaign against him, Fortichiari was readmitted in its ranks after the war, although he held secondary positions and was under a strict control of the party cadres, who were arguably weary of his intact charisma as the main figure inside an anti-Stalinist current.
He remained active until his death, apart a hiatus in 1965-1970, trying to collect the dissension against the party's official doctrine in the communist left movement.