By Wednesday, Mr. Salvini had moved beyond attacking Mr. Di Maio, saying that “another will arrive after him,” and that Five Star voters would continue to “make it pay” for its alliance with the Democratic Party.

“You will see Sunday,” he said, referring to the day of the regional elections.

Mr. Di Maio’s other former allies, including Prime Minister Giuseppe Conte, a law professor who was plucked from obscurity by Five Star to become Italy’s leader in 2018, were more measured in their reaction to Mr. Di Maio’s stepping back.

“If this will be his decision, I will respect him,” Mr. Conte told Italian radio on Wednesday morning, saying that it would be an initiative taken by Mr. Di Maio “with great responsibility.”

He would be sad, he added, “on a personal level.”

Mr. Di Maio’s retreat reflects a steep plunge in a trajectory that seemed to rise out of nowhere. A college dropout and former soccer stadium usher, he became an early favorite in the new Five Star universe. He won 189 votes on the party’s online nomination platform, enough to get his name on the national ballot and put him in Parliament when Five Star shocked Italy with strong results in 2013.

At age 26, Mr. Di Maio — who is often called by the nickname Gigi — became vice president of the lower house of Parliament. He served as a jacket-and-tie man to his party’s co-founder Beppe Grillo, a provocative comedian, as they campaigned around the country.

Behind the scenes, he was a trusted ally of Davide Casaleggio, a web entrepreneur and son of the party’s other co-founder, who many say is the party’s true power.