I grew up in such a place.
Everyone has choices to make. Absolutely the jerks are responsible for their own actions - regardless of what you wear on your person or on your bumper.
That said, you can choose to pick your battles. I prefer my mode of transportation not be put out of commission. The reality is that if my car was damaged (back in the vietnam war era in bright red rural Nebraska) by someone who decided they didn't like my bumper sticker, the cost to repair it came out of my pocket or that of my insurance company (after I paid the deductible). The chances of finding the guilty culprit were slim to none. Not to mention losing the ability to get to and from school or work for a few days while my car was being repaired.
So I chose not to display my politics on my car - because I would bear the consequences (regardless of my right to put whatever bumper sticker on my car I wanted).
I saved my political battles for situations in which those who might be prone to attack me would have to do so face to face (and be identifiable). So I might well wear a shirt bearing the same saying I wouldn't leave on my unprotected car.