Prosecutors in Naples charged former Sen. Sergio De Gregorio was paid $3.9 million to switch from the center left to the center right, the Italian news agency ANSA reported.
De Gregorio has admitted to receiving $2.6 million from Berlusconi, but denies it was bribe money.
After De Gregorio became a member of Berlusconi's party, the Senate ousted Prime Minister Romano Prodi in 2008 and installed Berlusconi as prime minister.
Berlusconi already faces other legal trouble. On Wednesday, a court in Milan upheld his four-year conviction on tax fraud. He is also on trial in Milan on charges he paid an underage Moroccan dancer for sex and abused his office in the resulting cover-up.