“I’m sorry for my son Luigi, who they are trying to attack,” he says, insisting his son knew nothing of his misdeeds and praising his “honesty, transparency and courage.” He demands that critics “leave my family alone” but argues that he had no choice but to hire workers off the books.

He signs off by apologizing, again, to his son and saying, “As a father, I can only encourage him to go ahead, not because he is my son, but because I believe he is doing good for this country, against everyone and everything.”

For years, “everyone” included relatives of the younger Di Maio’s political enemies.

Before Five Star came to power, its main target was Matteo Renzi of the Democratic Party, a former prime minister. Mr. Di Maio and the party’s army of online supporters seized on investigations into his father’s alleged influence-peddling as an example of the corrupt politics they promised to replace.

But Italian judges in October shelved a case against Mr. Renzi’s father, Tiziano, and fined Il Fatto Quotidiano, an Italian newspaper supportive of the Five Star Movement, for defaming him.

“I’m convinced that the sins of the father should not be visited upon the sons,” Mr. Renzi wrote on Facebook after the program about Mr. Di Maio’s father aired. “This is something I have always said, unlike Di Maio, who understands it only now.”

He went on to lament Five Star’s “campaign of hate” against his father, who, he wrote, has become reluctant to leave the house and worries about what his grandchildren hear about him. Mr. Renzi’s father recently wrote on Facebook that if he had done what Mr. Di Maio’s father had, Five Star “would already have launched an appeal on social networks for the return of the death penalty.”