Rosatellum used an additional member system, which acts as a mixed system, with 36% of seats allocated using a first-past-the-post voting system and 64% using proportional representation, with one round of voting. The Chamber of Deputies and the Senate did not differ in the way they allocated the proportional seats, both using the D'Hondt method of allocating seats. The new electoral law was supported by PD and his government ally Popular Alternative but also by the opposition parties Forza Italia and Lega Nord.
