Also on trial is Berlusconi associate Valter Lavitola, whom prosecutors said acted as a go-between between the ex-prime minister and former Sen. Sergio De Gregorio, Italy's ANSA news agency reported.
Prosecutors also allege Lavitola tried to bribe other senators who haven't been identified.
Berlusconi is accused of paying De Gregorio a multi-million-dollar bribe to leave the center-left and join the center-right, a switch that helped undermine the government of Romano Prodi during 2006-2008, ANSA said. Prodi's government collapsed after it lost support in the Senate, leading to new elections Berlusconi won.
In a plea bargain, De Gregorio admitted he didn't declare some of the money he received to tax authorities and was sentenced to 20 months in prison. He did not serve time because sentences under two years are automatically suspended, ANSA said.
The case is one of several legal woes for Forza Italia leader Berlusconi, who was tossed out of the Senate last year after a tax fraud conviction was upheld. He is appealing two other convictions -- one on paying for sex with an underage prostitute and then abusing his office in a cover-up attempt, and the other involving publication of a transcript of an illegal wiretap.
Berlusconi has maintained he has been targeted by a politically motivated judiciary.