NAHAL OZ, Israel — Benny Gantz, Israel’s former army chief, is campaigning to lead his country as the clean, moral alternative to Prime Minister Benjamin Netanyahu, who faces indictment in a corruption scandal.

But less than a month before the election, Mr. Gantz found himself fielding seamy-sounding questions late Friday about whether he had committed adultery, opened himself up to possible extortion or sandbagged his political allies to advance his own career.

The awkward news appearance here at a kibbutz near the Gaza Strip followed a report on Israeli television that Mr. Gantz’s cellphone had been hacked by Iranian intelligence after his entry into politics in December and that personal and professional information had been stolen.

Mr. Gantz retired from the military in 2015, so the issue is not whether classified information might now be in enemy hands, but that he could be vulnerable to blackmail if elected. The hack threatened to set back his bid, which polls have shown has the strongest chance of any opposition politician to depose Mr. Netanyahu. With rumors flying about what might have been harvested from Mr. Gantz’s cellphone, his party, the Blue and White, issued a statement saying the stolen data included “no security information, no embarrassing videos, and he was never a target of blackmail.”