Italy's outgoing Prime Minister Silvio Berlusconi refused to support the incoming centre-left government's candidate for new state president in Monday's first ballot, guaranteeing a stalemate in a vote in parliament.
Incoming Prime Minister Romano Prodi - who cannot take office until a new president is elected - had proposed 80-year-old Giorgio Napolitano as a compromise after Berlusconi said he would not accept a more high-profile leftist politician.
The 1,010 "grand electors", made up of lawmakers and regional representatives, began voting by secret ballot at around 4:25 pm (1425 GMT) to choose a successor to President Carlo Azeglio Ciampi, whose 7-year term expires this month.
But shortly before the vote, Berlusconi said his centre-right bloc would vote instead for his closest political aide, Gianni Letta, in the secret ballot of parliamentarians and regional government representatives.
Without the support of a large chunk of the centre right, Prodi will be unable to secure the two-thirds majority of 674 needed to elect a president in the first three rounds of voting. Acknowledging they would not be able to push through their candidate in Monday's vote, Prodi's bloc said the centre left would not bother to write in the name of Napolitano on their voting forms but instead cast blank ballots.
That would mean he could remain a candidate in further rounds of voting without being tainted by a string of defeats.
Prodi said the centre left had decided on the tactic while it waited for more signals from the centre right on the possibility of agreeing on a candidate.
By the fourth round, which would happen on Wednesday if the stalemate continues, a simple majority is sufficient, meaning Prodi may be able to push his candidate through against the opposition's wishes.
Whether that candidate remains Napolitano, an 80-year-old former parliamentary speaker and interior minister from the Democrats of the Left (DS) party, remains to be seen.
Members of DS, the biggest party in Prodi's coalition, initially wanted its chairman, the 57-year-old former prime minister Massimo D'Alema, elected to the country's top job, but he was rejected by the centre right as being too partisan.
Some analysts have suggested that Prodi only put forward Napolitano as a kind of "stalking horse", intending to revert to proposing D'Alema if Napolitano does not get broad backing.