By Steve Scherer
ROME (Reuters) - The unity of Prime Minister Matteo Renzi's party and the strength of his alliance on reforms with opposition rival Silvio Berlusconi face a test on Thursday with a parliamentary vote to choose a new president.
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 will hold a first round of voting at 3 p.m. (1400 GMT). A president is not likely to emerge from the first vote, when a two-thirds majority is needed, but in the fourth or fifth round - probably on Saturday - when only a simple majority is required.
After two days of consultations with political factions, Renzi is said to have chosen Sergio Mattarella, a judge on Italy's constitutional court and a former defence minister for the centre-left, as his candidate, Democratic Party (PD) deputy secretary Lorenzo Guerini said late on Wednesday.
But Berlusconi will not back Mattarella, Italian media have said. This sets up a possible showdown or opens the way for a last-minute compromise in voting by secret ballot that has shades of intrigue and unpredictability reminiscent of the papal conclaves which take place across the Tiber River.
Among the other names circulating as possible presidents are Romano Prodi, a two-time prime minister and former European Commission president; Giuliano Amato, a former prime minister and finance minister; Pier Carlo Padoan, the current economy minister; and Anna Finnocchiaro, a former minister.
The 89-year-old Giorgio Napolitano, who resigned as president 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 zone debt crisis. Napolitano appointed three 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 to push through institutional reforms with Berlusconi has broken down, 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 and loosening austerity, a political crisis in Italy would compound uncertainty in the euro zone.
The left of Renzi's Democratic Party (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.
And the PD leftists are seeking to undermine the smooth-talking Renzi, who took over the government by pushing out party rival Enrico Letta. Many commentators expect the eventual formation of a PD breakaway party.