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, due to a manslaughter conviction.
"Italy has developed a two-bloc system. We now have a three bloc system!" Walston said, referring to Grillo's shock success.
"That might work in a country like Austria, or like Germany" where there aren't such marked differences between coalitions. But in Italy, he said, "the personal, policy and ideological differences are too big."
Most analysts believe Bersani would seek an alliance with center-right Monti to secure a stable government, assuming parties gathered under Monti's centrist banner gain enough votes.
While left-leaning Bersani has found much in common with Monti, a large part of his party's base is considerably further to the left and could rebel.
A key Monti ally called the result "totally negative" and had an even gloomier assessment for his nation.
"As far as Italy goes," said Gianfranco Fini, "I fear the worst is yet to come."
Barry reported from Milan.
Copyright 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.