Generally speaking, the political argument against is "If you do anything that gives even the appearance of curtailing police powers in any way, we will create the illusion of a crime wave, attribute it to you, and absolutely hammer every form of media with that narrative." (If, God help you, you manage to pass a law that actually curtails police powers, they'll generally act in open defiance instead of doing their jobs.)

The opponent here isn't other citizens, it's the police and their organizations (police benevolent associations, police unions, etc.) which wield considerable political power, and to an extent also legal power (since they collectively have nearly full discretion over what laws are enforced and by what means, to the extent they can cooperate amongst themselves).