* Game show-style format for primary debate
* Party leader Bersani faces challenge from rival Renzi
* Centre-left leads in opinion polls ahead of 2013 vote
By James Mackenzie
ROME, Nov 12 (Reuters) - Italy's centre-left rivals Pier Luigi Bersani and Matteo Renzi faced off on Monday in a game show-style television debate to help decide who will run in next year's election as candidate to succeed Prime Minister Mario Monti.
The debate, before a primary on Nov. 25, comes at a time of exceptional uncertainty in Italian politics, with neither of the traditional big parties clear about who will lead them nor even what electoral law will apply at the ballot box.
Bersani, the 61-year-old leader of the Democratic Party, Italy's largest centre-left force and Renzi, the 37-year-old mayor of Florence who wants to drive out the party's old guard, are the front-runners in the race to lead the left in the election expected in April.
After a genteel encounter which was relatively light on specific policy proposals, it was not immediately clear who made the bigger impact with potential voters.
The Democratic Party, which supports Monti's unelected government, consistently leads in opinion polls but uncertainty about rules governing the election makes it unclear whether that will translate into a parliamentary majority next year.
Bersani, the favourite to win the primary, according to most opinion polls, struck a reassuring tone in keeping with his status as the senior figure in the race. "I don't ask you to like me, I ask you to trust me," he said.
Renzi, wearing the kind of purple tie once favoured by former British Prime Minister Tony Blair, played up his image as a moderniser, which has won him sympathy and annoyance in equal measure from different parts of the party.
"I think I don't just represent hope, but the future," he said at the conclusion of the debate which covered questions ranging from Europe to labour market reform, carmaker Fiat and gay marriage.
Both said they would improve on but not scrap labour and pension reforms made by Monti and both pledged to uphold budget commitments by his technocrat government to help control Italy's towering public debt.
As well as the two favourites, Nichi Vendola, the openly gay head of the Left, Ecology and Freedom party and governor of the southern region of Puglia also took part in the debate along with two other regional politicians but none of them is expected to win the primary.
According to an instant online poll after the debate by the Quorum Institute for the daily La Repubblica newspaper, Bersani was judged more convincing by 33 percent of the 2,700 potential voters surveyed, just ahead of Renzi on 31 percent.
Vendola and the two other outsiders, Bruno Tabacci and Laura Puppato, were well behind.
The debate, held by cable station Sky Italia rather than state broadcaster RAI and stged in a studio normally used for the talent show "X-Factor", was a rarity for Italian politics and borrowed heavily from the U.S. presidential campaign.
There was plenty of mockery on Twitter of the game show format, complete with buzzers and flashing lights but the novelty of the debate highlighted the desire of many in politics to re-connect with disillusioned Italian voters.
Traditional politics has been deeply shaken by the runaway success of the anti-establishment 5-Star Movement of the comic Beppe Grillo, with his caustic attacks on the rampant corruption and waste which has corroded public life. (Editing by Michael Roddy)