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“The first is the big surprising increase scored by the 5 Star Movement, and the other is the disappointing result” of Mr. Monti’s coalition, said Massimo Franco, a columnist with Corriere della Sera.

Turnout was 75 percent, down nearly 6 percentage points from the rate in the last national election in 2008. Experts say a low turnout will hurt the mainstream parties. Usually around 80 percent of the 50 million eligible voters go to the polls.

Under Italy‘s complex electoral law, how the upper chamber’s seats are divvied up depends on how the candidates do in Italy‘s regions, since the more populous regions, such as Lombardy, get a greater share of the seats.

Whether the center-left takes Lombardy might well decide if the coalition could stitch together a coalition with a workable majority in the Senate, as well as in the lower Chamber of Deputies, where the regional factor doesn’t exist.

Mr. Berlusconi, who was forced from office in November 2011 by the debt crisis, has sought to close the gap by promising to reimburse an unpopular tax — a tactic that brought him within a hair’s breadth of winning the 2006 election.

Mr. Monti, respected abroad for his measures that helped stave off Italy‘s debt crisis, has widely been blamed for financial suffering caused by austerity cuts.

Mr. Grillo’s forces are the greatest unknown. His protest movement against the entrenched political class has gained in strength following a series of corporate scandals that only seemed to confirm the worst about Italy‘s establishment. If his self-styled political “tsunami” sweeps into parliament with a big chunk of seats, Italy could be in store for a prolonged period of political confusion that would spook the markets. He himself won’t hold any office because of a manslaughter conviction

Most analysts believe Mr. Bersani would seek an alliance with center-right Mr. Monti to secure a stable government, assuming parties gathered under Mr. Monti’s centrist banner gain enough votes.

While left-leaning Mr. Bersani has found much in common with Mr. Monti, a large part of his party’s base is considerably further to the left and could rebel.

• Colleen Barry reported from Milan.