Nothing short of a felony would lead to a Democrat losing an election here but people in neighboring jurisdictions read about what happens here and politicians use that to foment fear, uncertainty, and doubt. Suburban voters are the swing in these purple states and you better believe these minivan moms are motivated by what is or isn't happening in their neighboring cities. The pushback is an inevitable consequence of trying to introduce such a radical idea to the mainstream. The Overton Window is a thing.

Why am I sure that this was the key issue? One reason is that there is literally nothing else to point to. We had four years of dog shit governance but the loss of control last summer is the only new thing to add to the equation. We also know that DTP polled extremely poorly. Opposition polled in the neighborhood of 58% which is massive. The Republicans were successful in painting the DTP label on Democrats, even those who opposed it and were only inclined to relatively minor reforms. Sure policing is a local topic but bottom-up is as good a reason to vote a single ticket as top-down. There was a lot of ambivalence for both Trump and Biden and in that situation, local topics can move the needle.

Are there other issues that influence voters? Sure, but this is one of the few that is separable from their core identity. You don't have to be a Democrat to support defunding the police (and in fact it was Democrats who instituted many of the policies that today's Democrats are now objecting to). Compare to something like abortion rights where a Democrat could not possibly get the nomination while opposing that view.