During these years he became a member of the Historical Far Left, the classical radical movement active in Italy in the second party of the 19th century.

In 1898 after the death of the left-wing leader Felice Cavallotti, Sacchi became the new head of The Extreme and started a process of modernization that ended in 1904, when he officially founded the Italian Radical Party.

Sacchi abandoned increasingly left-wing ideologies, switching the PR into a more moderate party.

In 1910s Sacchi implemented a politician alliance with the dominant leader of that decade, Giovanni Giolitti, who led the centrist Liberal Union.

In 1916 Sacchi was appointed again Minister of Justice, in the cabinets of the liberals Paolo Boselli and Vittorio Emanuele Orlando.