“This is what happens,” he said. “They want to discredit what you are saying, so they try to bring you down and turn you into a criminal. They tried to undermine my credibility, and now the president is doing the same thing.”

Mr. Serpico, who was profiled in a 2017 documentary and in another film this year, is a Brooklyn native who joined the Police Department in 1959 and grew increasingly disillusioned by the number of officers “on the pad,” or accepting bribes and payoffs from criminals, bookies and the like.

“It wasn’t about just taking a freebie,” he said. “It was cops doing the same thing that they were locking people up for.”

He steadfastly refused to take such money, he said, and in 1967 began complaining to high-ranking officials at Police Headquarters and City Hall.

They took little action, and he became a pariah in the department, he said. He considered quitting and simply walking away from the issue, a familiar decision for many whistle-blowers he has spoken to.

“At some point, you have to decide, ‘If I go up against the system, I better be ready to put my life on the line,’” said Mr. Serpico, who wound up taking his story to a Times reporter, David Burnham.

Mr. Burnham wrote a front-page article that helped prompt the Knapp Commission hearings in 1971 on police corruption, which, with Mr. Serpico’s testimony, helped expose pervasive wrongdoing and sparked the biggest shake-up in the department’s history.