The only, only bumper stickers I put on my truck are magnetic in nature and can be removed at my discretion. They, 99% of the time, have nothing to do with politics.
I have a magnetic sticker as well, a "13.1" oval from a race I did last October. A political sticker would probably be safe where I work - a defense contractor - and at church. But at the mall or a shopping center, I'll pass. Why tempt an idiot to do what they are?
I've seen some DUmmie cars, I can guarantee you they are either covering rust holes or holding something important on.
I've also noticed the more stickers on the car, the more liberal the driver.
I have seen a couple of cars held together by bumperstickers of a conservative POV, but most such I see are Lib/Prog, probably 5-to-1.