Rome's Mayor Walter Veltroni is heading for a landslide victory in a vote to lead Italy's new centre-left Democratic Party (PD), exit polls indicate.
The popular Rome mayor is seen by many as Mr Prodi's successor
They give Mr Veltroni, 52, nearly 75% of the votes in Sunday's election.
The party was born after a merger of the Democrats of the Left and the Margherita (Daisy) Party.
The PD's supporters hope it can boost the flagging fortunes of Prime Minister Romano Prodi's centre-left coalition. He is expected to quit politics in 2011.
The PD also aims to become Italy's largest political grouping, overtaking the right-wing Forza Italia of former prime minister and now opposition leader Silvio Berlusconi.
Exit polls give Mr Veltroni's closest rival, Families Minister Rosy Bindi, only about 14% of the vote.
Enrico Letta, undersecretary in Mr Prodi's office, was third with some 11%, while two other contenders won less than 1%.
About three million centre-left supporters took part in the poll.
Casting his vote, Mr Veltroni said: "It's a fantastic day for Italian democracy. We have in these primaries chosen to create a new party - a choice absolutely unique and unprecedented in European politics."
If confirmed, the margin of Mr Veltroni's victory would make him the prime candidate to replace Mr Prodi.
Mr Prodi has been pushing for the creation of the PD, in a plan to simplify Italian politics and stabilise his disparate centre-left coalition.
The PD's founding congress is due to be held on 27 October.