Berlusconi, 76, told an Italian talk show this week he will marry Francesca Pascale, 27, a former showgirl, after he told an interviewer last month he will run for a fourth term as prime minister.
Political commentator Vittoria Sgarbi called the engagement an campaign ploy, made in an effort to boost his popularity amid the negative news swirling around the wealthy businessman, the Italian news agency ANSA reported.
Berlusconi resigned as prime minister more than a year ago at the height of Italy's debt crisis.
"Berlusconi and Pascale have been together for at least a year and a half, and this [sudden announcement] is obviously planned for his campaign," Sgarbi said. "At this point, it's best to become pregnant," to enhance Berlusconi's family image.
"It's the story of a girl who finds a powerful man, that takes care of her."
CNN reported average Italians are expressing varied opinions about the December-June relationship.
"What can a man of 76 do with a 27-year-old girl?" a butcher asked.
"If it's fine for him an also for her, it should be fine for everyone," said a shopper.
Berlusconi, who has been convicted of tax evasion, is fending off criminal charges of paying for sex with an underage prostitute. He also is accused of embezzlement, fraud and bribery.