ROME — Silvio Berlusconi’s center-right political movement, a powerful force in Italy, split Saturday, dealing another blow to the former prime minister as Italian lawmakers prepare for a final vote this month on whether to expel him from the Senate.

The rupture in Mr. Berlusconi’s People of Liberty Party signals a new, if uncertain, chapter in Italian politics, with some analysts predicting rising levels of antigovernment populism.

The breakaway group was led by Mr. Berlusconi’s longtime protégé, Angelino Alfano, who announced that he and other former lieutenants would refuse to join the rebranded political party, Forza Italia (or Go Italy), that Mr. Berlusconi unveiled Saturday.

“I am here to make a choice that I never thought I would make, not to join Forza Italia,” Mr. Alfano said in a statement released Friday night. He announced the formation of the New Center-Right, a group expected to siphon away as many as a third of those lawmakers in Parliament previously committed to Mr. Berlusconi.