Giorgia Meloni broke with ex-P.M. in 2012 to form Brothers of Italy party.
A rebellion has left the ex-P.M. facing most serious challenge to his authority in 20 years.
Former P.M.'s fate could be sealed within weeks as key committee set to begin hearings before full vote.
European stocks fell on Wednesday as a U.S. government shutdown continued to rattle investors.
Three-time premier took the floor in an unexpected address after Letta's impassioned plea.
Senior figures in three-time P.M.'s center-right movement urge lawmakers to back centrist Premier Enrico Letta.
Relations with president have soured since ex-P.M.'s fraud conviction.
Expectations that a rebellion within Berlusconi's party may avoid snap elections curbs losses.
About 20 Italian center-right lawmakers may break with ex-P.M. over Letta dispute.
Investors shun Italian government bonds and shares.
