* Centre-left leader Bersani resigns over failed candidates
* Fifth vote for president expected to produce no winner
* Italy plunging deeper into political chaos
* President Napolitano could be asked to serve second term
Italy's divided parliament made a fifth attempt to elect a president on Saturday after a split over candidates flung the main centre-left alliance into disarray and forced its leader to resign, plunging the country into deeper political chaos.
Pier Luigi Bersani said late on Friday he would quit as Democratic Party (PD) leader as soon as a new head of state was elected, after party rebels sabotaged two separate candidates he had proposed. The PD's president, Rosy Bindi, also resigned.
A new president to succeed President Giorgio Napolitano, whose term ends on May 15, is needed to help broker a solution to political deadlock that has dragged on since an inconclusive February election.
But chaos and disunity in the largest party in parliament heightened uncertainty over how a government could eventually be formed.
Italian media speculated that they wanted to ask him to serve a second term even though the 87-year-old president has insisted on several occasions that he would not do so.
As the fifth ballot got underway on Saturday, it already seemed certain to produce no new president. PD electors had not declared how they would vote after a fifth of their ranks rebelled and failed to back Bersani's candidate on Friday. A previous candidate had also failed to pass in the secret ballot.
A candidate must gain 504 votes to win, out of a total of 1,007 electors made up of Italy's two houses of parliament and 58 regional representatives. Two rounds of voting are held each day.
The centre-left's junior coalition partner, Left Ecology Freedom (SEL), said it would back left-wing academic Stefano Rodota, the candidate backed by the anti-establishment 5-Star Movement who won 213 votes in the fourth round.
Caretaker Prime Minister Mario Monti, leader of the centrist Civic Choice that commands 70 votes, said his party would back interior minister Anna Maria Cancellieri, while the centre-right alliance led by Silvio Berlusconi said they would not use their 270 votes. Abstentions do not reduce the required majority.
The transformation in the fortunes of Bersani, who three months ago seemed the likely next prime minister, underlined the uncertainty over how a deeply divided political class can implement much-needed reforms to tackle an economy that has stagnated for the last 20 years.
There will now be a leadership battle in the PD, founded to unite a range of smaller leftist and centrist parties in 2007.
As electors entered parliament to cast their vote, they were heckled by protesters supporting the 5-Star Movement, who shouted in support of Rodota.
The dramatic success of 5-Star, which ballooned to command over a quarter of votes in its first national election, reflected the anger of Italians enraged by economic hardship and political corruption.