If carried out, the resignations would spark a crisis in Premier Enrico Letta's fragile, 5-month-old left-right hybrid government.
Senate President Renato Schifani told TG4 news today that 87 of the 91 senators of Berlusconi's People of Freedom party had turned in letters of resignation pending the October 4 vote.
Senators will decide then whether to strip Berlusconi of his seat following his tax fraud conviction and four-year prison sentence.
A 2012 law bans anyone sentenced to more than two years in prison from holding or running for public office for six years.