* Florence mayor Renzi wants to kick out centre-left old guard
* Young, effective TV performer appeals to many on centre-right
* Critics say he is image politician, more style than substance
By Gavin Jones
ROME, Nov 22(Reuters) - Matteo Renzi, the fresh faced mayor of Florence who wants to shake up Italy's ossified political establishment, is slick, dynamic, and above all young.
Renzi, 37, says that only by picking him as their leader can centre-left voters finally "scrap" a discredited party hierarchy that has failed for two decades to counter the political dominance of former Prime Minister Silvio Berlusconi.
In two months of hectic campaigning during which he has toured Italy's cities in a camper van, Renzi has shamelessly played the youth card against his main rival, the 61 year-old leader of the Democratic party Pier Luigi Bersani.
Most polls suggest that Renzi faces an uphill struggle against Bersani to win Sunday's primary ballot to become centre-left leader but if he succeeds, he would have little difficulty in winning the national election, due in the spring.
This is because Renzi, with his middle-of-the-road, market-friendly views, appeals to centre-right supporters almost as much as centre-left ones. Unlike the former communist Bersani, he would be expected to win over millions of disaffected former Berlusconi voters.
While official campaigning for the primaries only began in September, Renzi effectively launched his bid for the centre-left leadership a year ago at a snazzy U.S.-style convention in Florence which he called the Big Bang.
Since then he has been a constant thorn in the side of the leadership of the Democratic party (PD), by far the largest group on the centre-left, sniping at the old guard represented by Bersani and other party chiefs.
He has become a household name thanks to prolific rallies and television appearances, usually with white shirt sleeves turned up and jacketless, giving him a studied image of youth and dynamism.
Known as the "rottamatore" ("scrapper"), due to his desire to get rid of the old PD bosses, Renzi's confrontational stance has made him many enemies in the party.
Tensions have been raised by his claim that the complicated voting procedure for the primaries was deliberately created by the party to limit the turn-out to hard-core PD voters. His staff even warned this week of the risk of vote rigging at the ballot, which is all internally organised by the PD.
His eloquent, quick-fire TV style makes a striking contrast to the dour Bersani, but many PD politicians accuse him of being all style and no substance, an ambitious pragmatist who would be just as comfortable heading up the centre-right.
Renzi boasts that thanks to his efficient administration as mayor since 2009 Florence has been one of the few cities in Italy that has reduced local taxes, an example he would transfer to the national level.
A former boy scout who began his career with a now defunct Catholic centrist party, Renzi says he would continue the reform path of technocrat Prime Minister Mario Monti, cutting spending and selling state assets to reduce the massive public debt.
He says he would govern with just 10 ministers and cut Italy's bloated political apparatus by reducing the number of lower house deputies and replacing the Senate with a smaller chamber representing the regions, with limited legislative powers.