* Renzi made failed challenge to Bersani last year
* Could be difficult foe for Grillo
* New poll shows Renzi as favourite for premier (Adds new poll)
By Barry Moody
ROME, March 8 (Reuters) - The big winner in Italy's election was populist leader Beppe Grillo, but young Florence mayor Matteo Renzi is emerging as a less obvious beneficiary of a huge protest vote that threatens to destroy the old political system.
Renzi, a telegenic, 38-year-old American-style politician who toured Italy in a camper van before Grillo did the same in his campaign, is the opposite of colourless centre-left leader Pier Luigi Bersani, the election's biggest loser.
Whereas Bersani's oratory can leave an audience yawning, Renzi is an electric, fast-talking and articulate performer who paces the stage in his shirt sleeves pushing just the right buttons to whip up a crowd.
He has already demonstrated an acid wit in poking fun at Grillo, a shaggy-haired comedian more than 20 years his senior who mobilised public rage against Italy's discredited politicians to make his party Italy's biggest in the vote.
Bersani's drab campaign threw away a 10-point opinion poll lead to just scrape victory in the lower house of parliament ahead of Silvio Berlusconi's centre-right, but fall short of controlling the Senate - essential for government. No other party has a majority either.
Now Renzi is waiting for his moment as Bersani fights for survival, desperately trying to reach an agreement with Grillo that will allow him to form a minority government with backing from the anti-establishment 5-Star Movement.
With Grillo pouring insults on Bersani at every opportunity, the 61-year-old centre left leader looks to have only a tiny chance of success, despite near unanimous backing from his Democratic Party (PD) leadership this week.
The market-friendly Renzi challenged Bersani in party primaries back in November, threatening to "scrap" Italy's old leadership.
He was roundly beaten as the party machine mobilised PD cadres to support the leader. But Bersani's terrible election campaign has opened the way for a comeback.
An opinion poll on Friday showed Renzi was by far the favourite candidate for prime minister among Italian voters, with more than twice as much support as Grillo.
The SWG institute's poll showed the Florence mayor appealing widely across party lines with 28 percent support, compared with 14 percent for Bersani and 13 percent for Grillo. Berlusconi won 10 percent and outgoing premier Mario Monti six percent.
"To me it seems impossible that Bersani can succeed in forming a government and so frankly I think this attempt is an error which has no chance of seeing the light," a PD member of parliament close to Renzi's group told Reuters.
"Since there is a problem of relegitimising the party leadership, either we must confirm Bersani or replace him which many think is inevitable," said the parliamentarian, asking not to be named.
But Renzi is playing a cautious game, aware that if he jumps too soon he could blow his chances of both winning the party leadership and leading the centre-left to victory in another election that could be only months away - something which many pundits think is fully possible given his campaigning skills and appeal for even conservative voters.
So far he has insisted he is not challenging Bersani and expressed loyalty to his continuing leadership, but he has more and more openly attacked the party's failed election strategy and said the PD must be transformed.
"I tore my hair out after the election," Renzi said in a television interview this week. "We missed a penalty."
In an interview with Rome's il Messaggero daily on Thursday, Renzi said that his arguments during the primary election against Bersani last year had been proved right.
"The (national) election result showed that with some of my ideas, starting with the willingness to talk to disillusioned centre-right voters...perhaps we would have won the election."
Renzi's problem is that he is more popular among general voters than he is inside the party, where the traditional left wing with its roots in the old communist party views him with suspicion and sees him as too far to the right.
So timing is crucial.
"He is not going to jump now and risk getting burned before Bersani is really cooked," said Professor James Walston of the American University in Rome. He said Renzi would wait for Bersani to fail to form a government before making his play. "Renzi has the luxury of choosing his own time and not be forced," Walston said. "Bersani is not cooked yet."
Professor Gianfranco Pasquino of Bologna University said choosing Renzi would be a "fantastic move" for the PD.
But he added: "It depends if Renzi is willing to play this game which is very difficult. He may not want to burn himself at this point."
President Giorgio Napolitano, who has the task of trying to extract a viable government from the impasse, will start the process on March 19.
As leader of the biggest group in parliament, Bersani expects to get the first shot, but is widely expected to fail, leaving himself vulnerable to a party challenge.
The PD lawmaker said Renzi must make his bid at that point. "He cannot not be a candidate," he said.
He expected the party to call a congress to elect a new leader with the traditional leadership and left wing "Young Turks" backing Bersani or a younger leftwing candidate, but many rank and file members supporting Renzi.
Whoever wins would take the centre-left into a new election which most politicians expect as early as June but no later than a year from now.
Grillo sees another election as the way to complete the total destruction of the old political order and Renzi may be the centre-left's only chance of survival.
The Florence mayor is popular with old as well as younger voters - Grillo's biggest constituency - and is mobbed by well wishers everywhere he goes.
Renzi launched his bid for the centre-left leadership in 2011 at a slick U.S. style convention in Florence which he called the Big Bang. He is recognisable all over the country and at a recent Vatican concert dozens of elderly nuns and pensioners pushed forward to shake his hand as he walked past in his mayoral sash.
Much of his platform is close to Grillo's, including the removal of public financing from the political parties, which Bersani opposes, the abolition of parliamentarians' extravagant privileges and the speeding up technological innovation in which Italy lags many other developed countries.
Like almost everybody else he wants to change the electoral law which is one of the main reasons for the current mess, and pursue policies to revive the stagnant Italian economy.
Renzi is as outspoken against the political "caste" as Grillo and says the centre-left must appeal to a much broader group of voters than its core leftist backers.
He also shows much greater skill in attacking Grillo and his newly-elected 5-Star lawmakers than Bersani, who like many traditional politicians did not see how much of a threat the Genoese comic posed to the established order.
Renzi has mocked the 64-year-old comedian's stunt of dressing up in a hood and oversize aviator sunglasses to avoid talking to the press last weekend and has also poked fun at the embarrassingly eccentric ideas of some of his lawmakers.
"We need to talk about serious things, and instead we are all chasing the masked man," he said this week.
(Writing by Barry Moody; editing by Philippa Fletcher)