Surveys have found that they are deeply mistrusted by the public, often seen as doing the political bidding of whoever is in power. For decades, every president — including Mr. Moon — has vowed to take politics out of the agency, only to be later accused of using it to harass their political opponents or divert attention from domestic crises.

“South Korea is a veritable republic of prosecutors,” said Prof. Ha Tae-hoon of Korea University School of Law, who called the agency a “beast” that had defied democratic progress. Another law professor, Han Sang-hoon of Yonsei University, said the prosecutors’ rigid “command-and-compliance culture” made internal checks and balances all but impossible.

Few have condemned the prosecutors’ shortcomings more vigorously than Mr. Cho.

Articulate, good-looking and charismatic, he became a social media star while teaching at Seoul National University School of Law, calling for more social equality and high ethical standards for politicians. In a clip from a 2011 forum that recently went viral, he said South Korea needed leadership that “refuses to join hands” with prosecutors, and that a justice minister should reform their office.

“But I warn you that if the minister tries to do that, prosecutors can go after him, digging up dirt against him,” Mr. Cho said. “They are the kind of group fully capable of shaking and toppling the minister by spreading rumors against him.”

After Mr. Cho joined Mr. Moon’s staff in 2017 as his chief legal counsel, many saw him as a possible successor.

But his image soured drastically soon after Mr. Moon named him justice minister in August. News outlets began reporting on allegations of misdeeds by Mr. Cho or members of his family, including embezzlement and trying to destroy evidence.

Most of the accusations remain unsubstantiated. But his wife has been indicted on a charge of forging a certificate to help their daughter get into medical school. Mr. Cho has denied any lawbreaking by family members, but he acknowledged that his daughter had benefited from advantages denied to other students — a sensitive matter in a country where anger over economic inequality runs high. College students began holding rallies against him and calling him a hypocrite.