I am describing a problem where police unions hinder prosecution.

And they hinder prosecution in a way that essentially amounts to blackmail. They threaten that the police force will become less cooperative with the prosecutors office. An office that depends of the force to do it's job. A prosecutor should be able to look into charges against a police force without fear of retribution by said force. But they can't. And police unions are the spokespersons/ringleaders of the entire scheme.