By Steve Scherer
ROME (Reuters) - Italian Prime Minister Matteo Renzi proposed a senior judge to be the country's next president on Thursday, a move that his party welcomed but which may strain an alliance on reforms with opposition rival Silvio Berlusconi.
Though historically a largely ceremonial figure, the Italian head of state has important powers at times of political instability, a frequent occurrence in Italy, as he or she can dissolve parliament, call elections, and pick prime ministers.
Some 1,009 parliamentarians and regional officials started a first round of voting at 3 p.m. (1400 GMT), at which a candidate would need a two-thirds majority to win.
That is unlikely to happen, but from the fourth round -- probably on Saturday -- only a simple majority is required.
Renzi put forward Sergio Mattarella, a constitutional court judge and a former defence minister, as the candidate for his Democratic Party (PD) at a meeting of his party's electors, who unanimously accepted the candidacy.
Mattarella, whose brother was murdered by the Sicilian Mafia in 1980, would be "capable of guaranteeing Italy seven years of distinguished leadership," Renzi said.
It is still unclear whether Berlusconi will back Mattarella, who is not thought to be among his favoured candidates. In 1990 Mattarella resigned as education minister in protest over a government decree which favoured Berlusconi's media empire.
Since Renzi said he was not willing to select a compromise candidate, the choice of Mattarella could drive a political wedge between the two leaders whose alliance over electoral and constitutional reform has created friction in Renzi's PD.
INTRIGUE
On paper, Renzi has the numbers to get Mattarella elected from the fourth round, but the voting is done by secret ballot and has shades of intrigue reminiscent of the papal conclaves which take place across Rome's Tiber River.
Two years ago more than 100 PD electors failed to vote for the then party candidate, Romano Prodi, after having unanimously approved his candidacy in public before the ballot.
The previous president Giorgio Napolitano, 89, who resigned earlier 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 the vote.
Failure to seat a president in the fourth or fifth round would mean his authority over his party is wavering and the deal on institutional reform with Berlusconi is 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.
The left of the PD opposes the deal with Berlusconi, who they say should have been politically isolated after he was expelled from the Senate and barred from office due to a 2013 conviction for tax fraud.
PD leftists are also seeking to undermine the smooth-talking Renzi, who took over the government by pushing out party rival Enrico Letta. Many commentators anticipate the eventual formation of a PD breakaway party.
(Additional reporting by Giselda Vagnoni and Roberto Landucci; Editing by Mark Heinrich and Crispian Balmer)
