When I was a union ironworker we let people know who we supported. In local union elections among the rank and file and in presidential elections. In the former if you were on the losing side the payback could be significant but that's show biz.
I had a Kerry/Edwards sticker on my van. If I could have found an Obama/Biden sticker I would have carried it proudly. I had to settle for a button on my shirt that read "Bicyclists for Obama-Biden. This was not confrontational. It was meant like the previous examples to possibly influence others to vote that way. In the union we called that politicking.
My all time favorite political bumper sticker had a caricature of Nixon in profile with a long tongue sticking out of his mouth. Above that it said,"Lick Dick in '72".
Of non political stickers my current favorite says,"Keep honking, I'm reloading".
I don't care for religious bumper stickers or plates on automobiles, or for belt buckles with the name of a deity on them but hey, it's a free country.