* First round of voting fails to elect president
* Renzi proposes Sergio Mattarella as party candidate
* Has told party to vote for Mattarella in fourth round
* Proposal may strain reform deal with Berlusconi (Recasts with first round of vote failing to reach quorum)
By Steve Scherer and Gavin Jones
ROME, Jan 29 (Reuters) - Italian lawmakers failed to elect a new president in a first round of voting on Thursday, leaving Prime Minister Matteo Renzi hoping to push through his candidate only in a fourth round when the required threshold of votes is lower.
Renzi proposed constitutional court judge Sergio Mattarella, 73, to be head of state, a move which angered opposition rival Silvio Berlusconi and may strain the two men’s alliance over electoral and constitutional reforms.
Though a largely ceremonial figure, the Italian president wields important powers at times of political instability, a frequent occurrence in Italy, when he or she can dissolve parliament, call elections and pick prime ministers.
More than half of the 1,009 parliamentarians and regional officials cast a blank ballot on Thursday, preventing any candidate from reaching the required two-thirds majority.
There will be two more rounds of voting on Friday which are also expected to prove inconclusive. In the fourth round, due on Saturday, only a simple majority is required. It is then that Renzi hopes to secure Mattarella’s election with the backing of his Democratic Party (PD) and centre-left allies.
Berlusconi said his centre-right Forza Italia party would not back Mattarella and accused Renzi of breaking their pact on reforms by not proposing a jointly agreed candidate.
Mattarella, whose brother was murdered by the Sicilian Mafia in 1980, would be “capable of guaranteeing Italy seven years of distinguished leadership”, Renzi told the PD electors, who backed his choice unanimously.
Mattarella’s political roots are in Italy’s defunct Christian Democrat party. In 1990 he resigned as education minister in protest over a decree which favoured Berlusconi’s media empire.
Renzi’s choice of Mattarella has pleased Renzi’s left-wing critics in the PD. They strongly oppose his reform alliance with Berlusconi, who they say should have been isolated from politics after a 2013 conviction for tax fraud.
The candidate garnering most support in Thursday’s ballot was Ferdinando Imposimato, a senior magistrate proposed by the anti-establishment 5-Star Movement, who received 120 votes.
On paper, Renzi has the numbers to get Mattarella elected in the fourth round, but the voting is conducted by secret ballot and has shades of intrigue reminiscent of the papal conclaves which take place across Rome’s Tiber River.
Italy’s previous president, Giorgio Napolitano, 89, who resigned this month, used his powers to the full, intervening in 2011 to replace a scandal-weakened Berlusconi with ex-EU commissioner Mario Monti at the height of the euro debt crisis.
Napolitano appointed three unelected premiers in all.
The 40-year-old Renzi, who has been in power for less than a year, has a lot riding on this presidential vote.
Failure to seat Mattarella in the fourth or fifth round would mean his authority over his party is wavering and put the deal on institutional reform with Berlusconi in jeopardy, raising the spectre of an early national election.
With newly elected Greek Prime Minister Alexis Tsipras facing tricky negotiations with German-led European partners on renegotiating Greece’s debt, a political crisis in Italy would compound uncertainty in the euro zone. (Additional reporting by Giselda Vagnoni, Roberto Landucci and Isla Binnie; Editing by Crispian Balmer and Gareth Jones)