The Civil War crowd seems to lag behind in the bumper sticker department. This seems pretty boring but it's the liveliest I could find on the Web:
Maybe this blog should have one. "It's the primary sources, stupid" That's obscure, but pithy with clarity is hard to do. Hat tip to Neptunus Lex for a way out:
BTW, there's a comment on Lex's post that reflects my experience as well: "the cars with the largest number of, and usually most strident, bumper stickers on them often also have the most body damage (dented fenders, broken windows, body rust, etc). Or show obvious lack of maintenance or cleaning."
Draw your own conclusions.