By John Phillips

 ROME

Voting began in Italy's presidential election Thursday after Italian Prime Minister Matteo Renzi proposed constitutional court judge Sergio Mattarella to replace outgoing head of state Giorgio Napolitano.

Former prime minister Silvio Berlusconi has said his Forza Italia party will cast blank ballots rather than vote for Mattarella, the candidate of Renzi's Democratic Party.

Political experts believe Berlusconi may have to accept Renzi's choice to avoid a deadlock.

 Renzi is aiming to elect a successor to Napolitano in a fourth ballot Saturday when the number of votes needed to elect a new head of state will drop to a simple majority of 505 votes out of the total 1009 "grand electors." A majority of two thirds, or 673 votes, was needed in the first three rounds, which began Thursday afternoon.

  Renzi called middle of the road Mattarella, 73, a "man of legality, of the battle against the Mafia, of politics with a capital P." Mattarella is a constitutional court judge and former member of the once-dominant but now defunct Christian Democrat party.

 Mattarella's brother, also a Christian Democrat, was murdered by Cosa Nostra in 1980 while president of the Sicilian regional government, evidently because he refused to bow to the wishes of the Sicilian Mafia.

The post of president in Italy is mostly a ceremonial one. However, he can wield considerable powers when the country faces political instability. For example, he can dissolve the parliament or name a new prime minister.