Centre-left leader Pier Luigi Bersani on Wednesday proposed former Senate speaker Franco Marini as candidate for Italy's president, risking a dramatic split in his Democratic Party a day before voting begins.
Marini, a prominent Catholic and former head of the moderate CISL union, also got the backing of centre-right leader Silvio Berlusconi and the small centrist grouping of caretaker Prime Minister Mario Monti, but his election still appeared unsure.
Parliament begins voting on Thursday for the successor to President Giorgio Napolitano, whose term ends on May 15.
It will be up to the new president to end the political deadlock left by the inconclusive February election, either by persuading the parties to come to an accord that would allow a government to be formed or by calling new elections.
"Marini is the candidate who is best able to attract broad support," Bersani told a meeting of parliamentarians from his Democratic Party (PD). "He is linked to labour and social issues and is one of those who have built up the centre-left."
However, Marini faces opposition from within Bersani's own camp, with the 38-year-old mayor of Florence, Matteo Renzi, his main party rival, immediately saying he would not back him.
"We oppose this choice. Our parliamentarians will not vote for him," Renzi, who has at least 50 supporters in parliament, told the website of the daily La Stampa. Several other PD lawmakers immediately took a similar stance.
Renzi, who unsuccessfully challenged Bersani for the PD leadership last year and is by far the party's most popular politician, later said in a television interview that electing Marini would be "a disservice to the country".
He described the 80 year-old former unionist as "a candidate from the last century" who had no charisma or international standing and said it was unacceptable that his candidacy was based largely on the fact he was a well-known Catholic.
He also pointed out that he had lost his seat at February's election and said it was paradoxical that having failed to be elected as a senator he might now become head of state.
Marini's chances of success depend on how many PD rebels defy Bersani's party line. He will need a two-thirds majority in the secret ballot of lawmakers and regional representatives on Thursday.
Earlier on Wednesday the anti-establishment 5-Star Movement changed its candidate for president, saying it backed academic and left-wing politician Stefano Rodota after its original choice, television journalist Milena Gabanelli, pulled out.
Rodota, who was president of the PD's predecessor, the Democratic Party of the Left (PDS), in the 1990s, could potentially get significant backing from elements of the PD who do not want to do a deal with Berlusconi.
Other candidates often cited are former prime ministers Giuliano Amato and Massimo D'Alema, who are thought to have Berlusconi's backing, and former European Commission president and prime minister Romano Prodi, who does not.
Amato and D'Alema are both handicapped by perceptions that they are insiders too closely linked to the discredited traditional political elite.
A two-thirds majority of 1,007 electors from the combined houses of parliament plus 58 regional delegates are required to elect the president in the first three rounds of voting.
After that a simple majority is enough, meaning the PD could use the fact it has more deputies than any other party to force through a candidate with the backing of smaller groups if it can find any unity among its own ranks.
Voting begins at 10 a.m. (9:00 a.m. British time) and will proceed with two rounds per day until a president is elected.
As well as a ceremonial function, the head of the Italian state has a vital political role, a function Napolitano had to carry out during the 2011 financial crisis when he appointed Mario Monti to lead a technocrat administration.
(Additional reporting by Naomi O'Leary and James Mackenzie; Editing by Mark Heinrich and Jon Hemming)
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