None of the contenders gained more than 50 percent of the vote in the first round of the party's primary, ANSA reported Monday.
With most of the ballots counted, Bersani, a one-time industry minister, was expected to finish with 44.9 percent of the vote in the first round. Renzi was expected to be second with 35.5 percent.
Nichi Vendola, Pugalia governor and leader of Left-Ecology-Freedom Party, was projected to finish third with 15.6 percent. The party is poised to run as a coalition partner with the Democratic Party in next year's elections.
Two other candidates each finished with less than 3 percent of the vote.
Pundits told ANSA the primary winner has a good chance of being defeating Prime Minister Mario Monti, an independent, in the national election. Democratic Party candidates have been ahead of Monti in recent opinion polls.