The Five Star members have reiterated their intention to hold a referendum on abandoning the euro currency and held a meeting in Rome to iron out simmering differences between the party’s orthodox and pragmatic wings.

At the same time, many members fretted that the party was drifting away from its founding ethos — of voting on policies and candidates through direct, online democracy — and toward a centralization of power by Mr. Grillo, Mr. Casaleggio and Mr. Di Maio.

“We must be united and unanimous. Only one body, one soul,” Mr. Grillo wrote in his blog on Thursday, apparently worried his movement was troubled by traditional party problems. “We need shared ideas, not divisive opinions.”

The party, founded in 2009, tapped into the anti-elite resentments of those Italians who feel forgotten by the European Union and damaged by the introduction of the euro. It called for civic engagement, disdained corruption and, with its leaders’ willingness to indulge in invective and conspiracy theories, earned a profile as angry, webby and a little bit wacky.

For a while, the party was eclipsed by Mr. Renzi, a reformist. But having tied his fate to the referendum last Sunday on constitutional reform — and losing badly — Mr. Renzi resigned. Italy’s electoral law is under court review, so it may take months or longer before Italians can vote for a new prime minister. But Five Star is eager to hold immediate elections, as it is already polling at 30 percent in a multiparty race.

Whether the party has the maturity to govern is becoming an ever more pertinent question with consequences for the Continent’s fourth-largest economy and beyond. What is clear is that the party has replaced its early innocence with a killer instinct, with Mr. Grillo reversing his views on positions that his party once denounced as an undemocratic invitation to authoritarianism to improve his party’s electoral prospects. And Casaleggio Associates, the administrator of Mr. Grillo’s wildly popular blog, uses its affiliated websites to spread fake news that has damaged the party’s political enemies.