I’m not sure how much bumper stickers mean, but if you see a lot of Obama bumper stickers and no Romney, it might create overconfidence in the Obama people.
I’ve never put a political bumper sticker on my car but if I wanted to, I’d get a removable one and take it off while leaving my car in, say, the Wal-Mart parking lot.
Many dems are not particularly law abiding and might casually key my car on their way to cash in their food stamps.
This is reflected in the bumper sticker situation. I don't think it affects anyone's thinking.