The ex-premier is angry that senators from Letta's center-left party have refused to back a recommendation by his lawmakers that Berlusconi keep his Senate seat.
A 2012 law bans anyone sentenced to more than two years in prison from holding or running for public office for six years. This summer Italy's top criminal court upheld the media mogul's conviction and four-year sentence for tax fraud. The full Senate eventually will decide if he keeps his seat.
Berlusconi told reporters Thursday that a crisis in Letta's tense coalition could destabilize the recession-mired country and pledged his political support for now.