A court in Naples granted the Senate's request to be a civil plaintiff in the criminal case, the Italian news agency ANSA reported. The trial began Feb. 11.
Berlusconi, who has served longer than any other Italian prime minister since World War II, has been dogged by criminal investigations for years. He says he is the target of political prosecutions.
The Senate expelled him after he was convicted of tax charges.
In the current case, Berlusconi is charged with paying 3 million euros (about $4 million) in 2008 to Sergio De Gregorio, who was then in the Senate, to get him to change his political affiliation. De Gregorio's switch to center-right helped bring down the government of Romano Prodi, triggering an election that Berlusconi won.
De Gregorio faces 20 months in prison after admitting in a plea bargain that he failed to pay taxes on some of the payoff.