Pier Luigi Bersani, head of Italy's centre-left Democratic Party, looks set to win the first round of a contest to lead the left into a general election next year.
With nearly half the votes counted, Mr Bersani led his main rival, Florence Mayor Matteo Renzi, by 44% to 37%.
If neither reaches a 50% majority, they will face a run-off vote on 2 December.
Nichi Vendola, the head of the leftist Left, Ecology, Freedom party was in third place with 14% of the vote.
Elections expected in early 2013 will choose a successor to Prime Minister Mario Monti's government.
The centre-left alliance leads polls ahead of the general election. Mr Monti's successor will face the challenge of addressing Italy's deep-rooted economic problems.
Speculation has been mounting as to whether Silvio Berlusconi would run for a fourth term in office, with the ex-premier suggesting on Saturday that he was thinking about returning to politics.
His record tarnished by sex and political scandals, Mr Berlusconi resigned as prime minister last November, after MPs approved an austerity deal to help curb the debt crisis threatening the eurozone.