ROME — Pier Luigi Bersani punctuated his first day as the point man for Italy’s center-left parties by vowing that he would lead the coalition to victory in next year’s national elections.

“The next adventure is the government, a government of change,” Mr. Bersani said Monday, hours after winning hard-fought national primaries. He defeated Matteo Renzi, the mayor of Florence, 24 years his junior, who had campaigned on a platform of change and generational renewal.

Mr. Bersani, 61, who has been the secretary of the Democratic Party since 2009, ran as the favorite, with nearly the full support of the party apparatus and its elected officials. He easily defeated Mr. Renzi, winning nearly 61 percent of the vote.

But Mr. Renzi’s message of change rang forcefully with a sizeable chunk of the center-left electorate, with over one million supporting him. He also attracted a considerable number of mostly young center-right voters whose frustrations with Italy’s influential and pervasive gerontocracy obliterated party lines.