Political darling Luigi di Maio, 31, is the reassuring face of Italy's anti-establishment insurgency and is widely seen as having the vote in the bag.
The clean-shaven lawmaker has been openly groomed to run for the top job in the spring 2018 general election as an everyman able to win over average voters and financial markets alike.
“Di Maio is studying how to become prime minister; he behaves, speaks, walks like one. He scrutinises the rules of the game to try and beat the house,” Luiss University communications expert Alberto Castelvecchi told AFP.
The telegenic youngster went up against six totally unknown candidates and a low-profile senator in an electronic vote on Thursday that both amused and irked traditional parties and the country's mainstream media.
“The pathetic primary with Di Maio as a lone candidate (…) is not only a symptom of a lack of democracy” within M5S, wrote Il Fatto Quotidiano daily, which is usually the most sympathetic to the movement.
“It is also proof of the eternal immaturity, incompetence, inexperience and thrown-together nature of a movement that is getting bigger but not growing up,” the paper said.
M5S, which bases much of its appeal on fighting corruption, emerged as a major force in 2013 general elections and went on to win mayoral seats in Rome and Turin last year.
And despite a worse-than-expected performance at local elections in June, for months now it has been neck-and-neck in the polls with the centre-left ruling Democratic Party (PD), with recent projections showing it would win 28 percent in a national race.
Critics say that regardless of who is candidate for prime minister, 5-Star's outspoken founder, comic Beppe Grillo, will continue to determine how the party is run, crushing any dissent.
Grillo has promised to remain the “guarantor” of the movement's model of direct democracy.
Members are told whoever gets the PM candidacy and party leadership will be a mere spokesman, obliged to follow a political programme “for Italy, written by Italians”.
5-Star voters range from those with a passion for grassroots politics to those convinced it would be disastrous in power but believe the crisis would usher in a longed-for new political era, Corriere della Sera daily said.
The movement has always defined itself as on neither the political left nor right, though it was driven at the start by largely leftist ideals such as calls for a minimum wage, according to political watcher Vincenzo Latronico.
Its leaders have moved sharply across the spectrum since, often echoing the anti-immigration Northern League party.
Picking a PM candidate may thrill supporters, but the movement will still have an uphill struggle to reach high office.
It categorically refuses to form alliances with any other parties and under Italy's proportional electoral system would need to pocket 50 percent of the vote plus one ballot to obtain a majority in the upper house of parliament.
It would only need 40 percent of the vote in the lower house — but analysts say that looks like a tall order.
By Ljubomir Milasin