But the vast majority of those problem police are governed by liberal or progressive governments. Those police agencies should be models for the rest of the country and the world absent any involvement by the federal government.
Very true. You should think that LAPD or NYPD should be good examples how liberal cities are running great police departments but that’s clearly not the case.
Again, it's not that easy to blame "the democrats". The people at the top may go away in reelections when a Republican loses their seat, but the "base" structure, think the cops and their management that has been there for decades, will stay the same - and so will the organizational inertia.
To make it worse, a large part of the blame lies on police unions, e.g. in Minneapolis, whose leaders are hard authoritarian/right-wing and use all powers they have including staging expensive and often successful legal battles to keep "bad apple" cops in the service instead of holding them accountable.
Some of these cities have been under complete liberal democrat or progressive control for decades. These cities are operating under police contracts and policing standards set and enforced by liberals or progressives.