Italian MPs have failed in three attempts to elect a new president to replace Carlo Azeglio Ciampi.
Mr Ciampi has decided not to seek another term in office
MPs from the new centre-left coalition could not agree with opponents on a compromise, and no candidate managed to win the necessary two-thirds of votes.
Mr Ciampi, 85, is stepping down after seven years in office.
Only when the president is elected can centre-left leader Romano Prodi - who narrowly won the elections last month - start to form his new government.
The first three ballots in the vote for president require a two-thirds majority. Thereafter only a simple majority is needed.
A fourth round is expected to be held on Wednesday.
Mr Prodi said his bloc, with a slim parliamentary majority, would force through his candidate, Giorgio Napolitano.
"We have simply decided all together to vote for Napolitano," he said after meeting his senior allies.
Italy's head of state - basically a figurehead - is elected by 1,010 Grand Electors, including MPs from both houses and representatives of 20 local regional governments.
Mr Prodi put forward at the last minute a compromise choice - the 80-year-old life senator Mr Napolitano, a former communist.
Mr Napolitano replaced Mr Prodi's previous candidate, Massimo D'Alema, another former prime minister and leader of Italy's largest left-wing party, the Democrats of the Left.
Outgoing Prime Minister Silvio Berlusconi argues that the left is trying to grab all the highest offices of state, having only barely won the recent election.
He put forward the name of Gianni Letta, a former newspaper editor and his chief political adviser in the government that has just resigned.