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* 5-Star picks Stefano Rodota after first choice pulls out
* Leftist Rodota seen with more chance of success
* Still no clear favourite for president on eve of election
By Gavin Jones
ROME, April 17 (Reuters) - Italy's 5-Star Movement changed its candidate for president on Wednesday after the winner of an online ballot withdrew, adding to uncertainty ahead of a vote that is key to ending political stalemate since February's general election.
Less than a day before parliament starts to vote on a new president, parties are struggling to find any broad agreement on a candidate to succeed President Giorgio Napolitano, whose term ends on May 15.
The new president is the only person who can 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.
The 5-Star Movement, the only party to have named a candidate so far, said it was backing academic and left-wing politician Stefano Rodota after its original choice, television journalist Milena Gabanelli pulled out.
Unlike political outsider Gabanelli - who said she did not want to be president - Rodota could potentially get significant backing from elements of Pier Luigi Bersani's centre-left Democratic Party (PD).
Rodota was president of the PD's predecessor, the Democratic Party of the Left (PDS), in the 1990s and might emerge as a realistic presidential candidate if the PD chooses to seek a deal with 5-Star rather than Silvio Berlusconi's centre-right.
"The withdrawal of Gabanelli forces the PD to vote for Rodota without any ifs or buts, otherwise the centre-left will split into a thousand pieces," said Michele Emiliano, PD mayor of the city of Bari.
However, the PD is already deeply divided and others in the party would prefer to see a deal with Berlusconi, which they believe would leave more room for a government to be formed.
In that case, former prime ministers Giuliano Amato or Massimo D'Alema could have a stronger chance of election, although they are both handicapped by perceptions that they are insiders too closely linked to the discredited political elite.
Another name often cited is former European Commission President Romano Prodi, whom Berlusconi has already rejected.
A two-thirds majority of 1,007 electors from the combined houses of parliament plus 58 regional delegates is 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.
Voting begins at 10 a.m. (0800 GMT) on Thursday and will proceed with two rounds of voting per day until a president is elected.
As well as a ceremonial function, the head of 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.