ROME — For months, he has been shouting his way from piazza to piazza, drawing tens of thousands as he rails against tax collectors, corrupt politicians and financial speculation. And when he arrives in Rome on Friday for the final campaign rally of his “Tsunami tour,” Beppe Grillo, the Internet-savvy comedian turned populist rabble-rouser, may lead Italy’s third-most popular party.

In the final weeks of a campaign marked by widespread voter disillusionment and growing economic distress, and after Pope Benedict XVI’s decision to step down on Feb. 28 upended once-solid assumptions, Mr. Grillo’s Five Star Movement has surged ahead, surprising the experts almost as much as Pope Benedict did. Ahead of national elections on Feb. 24 and 25, the party’s “antisystem” message has drawn strong support from both right and left, buoyed by corruption scandals that have undermined voters’ faith in government.

With his fiery language, mop of gray curls, grass-roots campaigning and calls for a referendum on Italy’s staying in the euro zone, Mr. Grillo, 64, has helped blunt the gains made by Silvio Berlusconi, the former prime minister. Mr. Berlusconi’s People of Liberty party is now in second place in opinion polls but lacks potential coalition partners, making it all but impossible that Mr. Berlusconi, the ultimate campaigner, will ever govern Italy again, political experts say.

The center-left Democratic Party, led by Pier Luigi Bersani, a subdued former industry minister, is expected to place first, but it may not win enough seats to govern without help from an ally, probably the civic movement started by Mario Monti, the caretaker prime minister who stepped down before the elections after leading a technocratic government for the past year. Mr. Monti is respected by world leaders but has fumbled into fourth place as a candidate, seriously tarnishing both his image and the agenda of change he promoted.