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.