The main challenge now is whether Prodi's candidate for Senate president _ Franco Marini, a centrist and former labor leader _ can beat out the choice of outgoing Premier Silvio Berlusconi's coalition _ former Christian Democrat premier Giulio Andreotti.

The margin for Prodi's coalition is tighter in the Senate than in the lower Chamber of Deputies, which also begins elections for its president.

In a news conference Thursday, Prodi expressed the hope that his "majority would be solid" and stay compact.

Italian news reports said Berlusconi was expected to head to the presidential palace and turn in his resignation to the head of state after the leaders of both houses of the new legislature are elected.

Several rounds of balloting are expected before the winners emerges, with parliamentary officials saying the voting would likely stretch into Saturday.

Resignation by Berlusconi, who has refused to acknowledge Prodi's victory, would pave the way for President Carlo Azeglio Ciampi to give Prodi the mandate to try to form a government.

But Ciampi's own term in office runs out in mid-May, and he has indicated he would prefer that his successor, who will be elected by lawmakers, to tap the new premier.

Marini, 73, is a former leader of one of Italy's major labor confederations that has traditionally been supported by Christian Democrats. Andreotti, 87, was longtime leader of the Christian Democrats when they dominated politics in Italy for decades after World War II until corruption scandals toppled them from power.

Although Prodi's center-left coalition has the numbers to get Marini through, there is the danger that some of those lawmakers could defect to Andreotti in the secret ballot. Any such defection would seriously shake Prodi's coalition, which already has shown signs of fraying over which politician gets which post under his tenure.

But Andreotti's chances are hampered by the fact that the Northern League, and often xenophobic, anti-immigrant party within Berlusconi's coalition, said its senators would not vote for Andreotti.

Instead, the party said it would push for Roberto Calderoli, who was Berlusconi's former reforms minister. Calderoli was forced to quit the government in February after he wore a T-shirt with a caricature of the Prophet Muhammad on state TV.

Votes by Italians living abroad proved decisive in the race for the Senate, where Prodi won 158 Senate seats to Berlusconi's 156 by taking four out of six spots up for grabs abroad.