ROME (Reuters) - Italy's centre-left votes on Sunday for a candidate who will become front runner to succeed Mario Monti as prime minister after a general election next spring.
There are five candidates in the primary election but in reality the vote will pit Democratic Party (PD) leader Pier Luigi Bersani against youthful Florence mayor Matteo Renzi for leadership of an alliance which is well ahead in opinion polls.
In an often bitter generational struggle, Bersani, 61, represents the traditional post-communist leadership of the left while Renzi, 37, vows to "scrap" the old guard and lead a root and branch renewal of Italy's political system.
Recent opinion polls show Bersani, a colorless but canny career politician, pulling ahead of Renzi - whose shirt-sleeved U.S. style rally speeches and modernizing message have not been enough to convert traditional leftist supporters.
The party poll will remove one of the major elements of uncertainty dogging Italian politics ahead of the election to choose a successor to Monti's technocrat government.
Its outcome will likely not be known until after December 2 when a second round runoff will be held.
Polls currently have Bersani from 10-15 points ahead on just over 40 percent, well short of the 50 percent he needs for a first round victory. But he is likely to pick up the votes of third placed left winger Nichi Vendola, the openly gay governor of the southern Puglia region, in the second round.
While the fresh-faced, articulate Renzi, who comes across like an Italian version of Britain's Tony Blair, is much more popular across the general population than Bersani, he is far weaker among party supporters who will decide the primary.
Both men reject the idea strongly espoused by international markets that the respected and sober Monti should return after the vote to continue his economic policies.
With Silvio Berlusconi's centre-right People of Freedom (PDL) close to disintegration and slumping in opinion polls to less than half its support in the last election in 2008, the centre-left has a clear field to win the general election.
However there are many other undecided variables ahead of that vote including the dramatic rise of the anti-establishment 5-Star Movement, which is now running second in opinion polls, and the around 50 percent of Italians who tell pollsters they are either undecided or will abstain.
It is also still unknown what electoral system will be used for a vote now expected on March 10-11, with politicians squabbling for months over how to change an unpopular electoral law that allows party leaders to hand pick members of parliament.
If the much-despised "pigsty" law is not changed, the centre-left would benefit from one of the central planks of the current system, a huge bonus guaranteeing the biggest party 55 percent of the vote.
Otherwise, opinion polls suggest it could fall short of a workable majority in parliament and may need a post-election alliance, probably with centrists.
Supporters of the market-friendly Renzi say he would be a better bet to continue Monti's agenda than Bersani who they accuse of being a hostage to the radical left.
This is strongly denied by PD officials who insist they are committed to a prudent pro-European agenda but say that Italy desperately needs growth and greater protection for workers and pensioners who have suffered most from a biting recession.
The centre-left sees the publicity attracted by the primary election and the likely participation of around three million party and non party voters as a powerful weapon against the populist 5-Star Movement of comedian Beppe Grillo, whose runaway success has deeply shaken traditional politicians.
A U.S. style debate among the five candidates on November 12, in a television studio used for the X-Factor talent show, garnered record viewer ratings.
"It was a great decision to call the primaries. Everybody is talking about them. They put us in the centre of attention," PD deputy leader Enrico Letta told Reuters. "They were the only way to combat Grillo, the best antidote to him."
The centre-left is notoriously fractious and many supporters are haunted by the memory of their last government under Romano Prodi, which fell apart after two years in 2008.
Letta said the primaries had also restored unity to the centre-left. The five candidates in the debate kept to a genteel tone marked by more agreements than disagreements.
The format clearly favored the telegenic, fast talking Renzi, but Bersani held his own, projecting an image of calm statesmanship which pundits say appealed to many Italians.
However, critics say Bersani would be pushed to the left by Vendola's small Ecology and Freedom party, threatening Monti's heritage and pushing Italy's borrowing levels to the critical levels reached under Berlusconi a year ago.
Letta and other officials deny this, saying one of Bersani's greatest achievements has been a binding pact signed by all the primary candidates including Vendola to abide by majority decisions in a centre-left government. They add that the PD has provided crucial support for Monti, keeping social peace and containing trade union protests.
"We have supported the government through great difficulty with the unions," said PD official Lapo Pistelli.
Traditional leftists say the upstart Renzi appeals more to Berlusconi's former supporters than their own and he would split the party. They say he is all image and no substance.
Bruno Tabacci, another primary candidate, told Reuters: "Renzi seems not very honest intellectually. I would be ashamed of saying the things he says."
Editing by James Mackenzie and Philippa Fletcher