Mr. Casaleggio, 42, may then be Italy’s most powerful IT guy. Party dissidents and critics say that the uncharismatic Milanese management consultant, who is far from a household name even within Italy, is the bland Big Brother of an Orwellian party that could shake up all of Europe.
Five Star’s bylaws require it to use the web platform Mr. Casaleggio helped found and which shares a Milan address with his Milan-based company, Casaleggio Associates.
He and his supporters say that he volunteered it as a tool for the party, but documents recently revealed in the Italian press seem to show that he maintains absolute control over the association that manages the web platform and has the ability to know everything that transpires on it.
Party dissidents say that it is Mr. Casaleggio who, from behind the scenes, expels nonconformists and enforces a strict party line.
Italian authorities have raised concerns about who has access to the voting records and identities of individual members, and party dissidents have raised suspicion that votes over his platform are being manipulated, something that Mr. Casaleggio and his party deny.
Mr. Casaleggio declined repeated requests for an interview, explaining that he did not trust the news media.
His company has hosted, and earned advertising revenue from, websites littered with conspiracy theories and fake news, messages echoed across a vast network of supportive social media accounts.
Wary investors and political leaders in Europe are still unclear what the party’s position is on the euro, migration, vaccines, Russia and much else. A recent spate of scandals involving Five Star candidates have reinforced doubts about their qualifications to run the third-largest economy in the eurozone.
One of the co-founders of the Five Star Movement was Gianroberto Casaleggio, a visionary internet entrepreneur and Mr. Casaleggio’s father. It was supposed to be an answer to Italy’s corrupt back-room deals and the ossified hierarchies of its traditional political parties.
The party eventually adopted the web platform called Rousseau — after the 18th-century philosopher Jean-Jacques Rousseau — promoting it as a tool of direct democracy in which all clicks were created equal.
But since inheriting power over the party after his father’s death in 2016, Davide Casaleggio has consolidated control, say critics and party dissidents.
“He has the keys to everything, all of the infrastructure,” said Jacopo Iacoboni, the author of “The Experiment,” a critical exploration of the Five Star Movement. “He decides who runs and doesn’t run. He is the boss.”
Rank-and-file party activists say observers should not pay attention to the man behind the curtain. They contend that Mr. Casaleggio has no official role in the party and is only a management consultant.
“He’s a researcher and a technician,” said Massimo Enrico Baroni, a member of Parliament. “A poet and a visionary.”
Caterina Grimaldi, a city councilor from Voghera, said Mr. Casaleggio kindly housed Rousseau in his office space for convenience and to cut costs.
“He has nothing to do with politics,” she said, balking at the suggesting that he would ever abuse his position within the party. “I trust these people.”
The two politicians and other members of the party had just exited a private dinner on a recent rainy evening in Milan, where Mr. Casaleggio, unusually, held forth on his vision of web-based politics.
“Through the web we can try and resolve problems that we have never been able to tackle,” he proclaimed, wearing a stubble beard on his sharp chin and a checked blue suit.
He seemed happy to get off the stage, though he gamely mixed with attendees and party members waiting their turn to talk to him late into the night.
By the bar, he stood shoulder-to-shoulder greeting guests with Luigi Di Maio, a squeaky-clean 31-year-old who is the party’s official political leader, public face and, after receiving 30,936 votes on Rousseau, a candidate for prime minister.
Absent that evening was the party’s founding standard-bearer, the gleefully vulgar comedian Beppe Grillo, who has in recent weeks appeared to distance himself from the party by removing his wildly popular, and powerful, blog from the servers of Casaleggio Associates.
The break has prompted speculation about divisions within the party, with its “orthodox” faction unhappy about Mr. Casaleggio’s efforts to use Mr. Di Maio to transition from protest party to powerhouse. The movement has denied any internal tension.
The bilingual Mr. Casaleggio, whose mother was a British translator, competed in chess tournaments as a child and focused on e-commerce at Bocconi University’s business school in Milan.
At Kangaroo.it, a start-up to help IT professionals with job searches, the young graduate managed operations and was quiet, honest and constantly saying “okey-doke,” said the company’s founder, Franco Bondi. He now spends his free time scuba diving in cold lakes. The limelight is not for him.
His opponents have sought to draw him out. Matteo Renzi, a former prime minister who in 2016 was targeted by Russian-sourced fake news published on websites hosted by Mr. Casaleggio’s company, said Mr. Di Maio was “remote controlled” by a mysterious power “who hides behind software.”
The party’s dissidents agree that Mr. Casaleggio is at the controls.
Giovanni Favia, an early officeholder with the party from Bologna, said his suspicion that Casaleggio Associates was manipulating votes helped prompt his expulsion in 2012. Since then, he said, things have only gotten stranger.
“Casaleggio has an association that controls the movement, but the movement is a client of his,” he said, questioning whether the firm’s political connections have earned it more lucrative advertising or clients.
Lorenzo Andraghetti, another former party member, recalled that when he had a problem in the early days of the movement, Mr. Grillo told him to see Davide Casaleggio because he maintained connections with the local activists.
Mr. Andraghetti said Mr. Casaleggio carried out his father’s orders, including his own political demise in 2015. “He is the one who eliminated my nomination with a click,” he said.
Mr. Casaleggio is apparently not the only one with access to the party’s voting platform. Hackers have repeatedly breached Rousseau, raising concerns from the independent Italian Data Protection Authority about the private data and voting records of members.
One hacker claimed to have manipulated the vote for Mr. Di Maio and mocked Mr. Casaleggio by publishing his cellphone number and his password to the platform.
Mr. Casaleggio has responded by saying he is at work on building a bigger, safer Rousseau. To fund its web platforms, the party has levied a fee of 300 euros a month on newly elected party representatives.
That is not the only arrangement that has raised allegations of a conflict of interest, an accusation traditionally levied against Silvio Berlusconi, the former prime minister and media mogul, who says he is taking part in the elections mostly to stop the Five Star Movement.
But Mr. Casaleggio is looking toward a new Italian utopia.
“Let’s not wait until the future knocks on our door,” Mr. Casaleggio said from the stage in Milan, where attendees deposited in a closed box surveys saying what they hoped would be the priorities of Mr. Casaleggio’s new association. “Let’s build it ourselves.”