As scores of police officers, supervisors and union officials are investigated for fixing tickets in the Bronx, the Sergeants Benevolent Association — the powerful union for the Police Department’s 12,000 front-line supervisors — has started a campaign arguing that the practice, while widespread, is one of courtesy, not corruption.

Edward D. Mullins, the president of the sergeants’ union, has recorded an audio message calling on current and retired members of the force, across all ranks, to come forward with testimonials about the beneficiaries of ticket-fixing. He said he expected to find evidence that politicians, prosecutors, clergy members, business leaders, celebrities, athletes and others have been among those who have had tickets fixed, often with the help of top police officials.

The move is a striking public frontal assault on the investigation by the union, even though the inquiry itself has never been officially acknowledged by law enforcement officials, and no charges have been filed.

Mr. Mullins said in an interview that his aim was to highlight a culture of courtesy that had been the norm since the inception of the summons. But it could also serve to embarrass or even implicate public officials or others who have asked police officers to do them a little favor and make a ticket go away.