In the late 1980s he became a member of the Christian Democracy (DC) party and was elected city councilor in his hometown.

When the DC was later disbanded he joined the Italian People's Party (PPI) and The Daisy (DL).

On 16 June 2015 he was elected Leader of the PD in the Chamber of Deputies with 239 votes out of 291, replacing Roberto Speranza.

[2] In Autumn 2017, Rosato was the proposer of a new electoral law, which was named after him, Rosatellum bis.

[4][5] The new electoral law was supported by the Democratic Party and his government ally Popular Alternative, but also by the opposition parties Forza Italia, Lega Nord and Liberal Popular Alliance.