Tommasi became a union activist, and during World War I (1914–18) supported the left wing of the CGT.
[2] At the CGT's first postwar congress, held in Lyon from 15 to 21 September 1919, Tommasi was among the leaders of the minority, with Pierre Monatte, Raymond Péricat and Gaston Monmousseau.
[1] He was a delegate to the Third Congress of the Communist International in Moscow in June–July 1921, where he supported the Left current.
[1] Tommasi attended the congress in Saint-Étienne on 25 June – 1 July 1922 at which the syndicates, unions and federations that had been excluded from the CGT founded the Confédération générale du travail unitaire (CGTU).
He moved to Moscow, where he joined the Left Opposition led by Leon Trotsky.