But in a remarkable challenge to Berlusconi's authority, several allies balked and said they would instead support Letta's hybrid right-left coalition.
Italy's finances are in a critical state, pressing economic measures must be passed and Italy's president has insisted that a new electoral law be passed to avoid inconclusive results in any future vote.
Heading into the vote, the numbers were in flux.
Dissenting Sen. Roberto Formigoni said some 25 Berlusconi allies had signed on to support Letta, apparently enough to tip the balance in Letta's favor in the 321-member chamber.
Rather than highlight the divisions and defections, Berlusconi instead threw in the towel and changed course.