With Gabriele D'Annunzio, he coauthored the Charter of Carnaro, a constitution for Fiume. De Ambris provided the legal and political framework for the document while D'Annunzio used his skills as a poet to make the document more impressive; he was also the leader of a group of Italian seamen who had mutined and then given their vessel to the service of D'Annunzio.
Although linked to the beginnings of Fascism through his attitude in 1914 and the support he gave D'Annunzio, De Ambris became a vocal opponent of Partito Nazionale Fascista and Mussolini's dictatorship. He and D'Annunzio were asked by Fascist supporters to run in the elections of May 15, 1921, but both refused. Before the March on Rome, De Ambris depicted the Fascist movement as
"a filthy pawn in Mister Giolitti's game of chess, and made out of the least dignified section of the bourgeoisie"
His citizenship was withdrawn in 1926, and he had to flee for France. He continued attacking the regime through his writings (published as Lettere dall'esilio - "Letters from exile").
