The people who need to understand it the most -- i.e., liberals -- wouldn't get it, so you'd probably be wasting your money.
Try doing what I did. Get a blank magnetic bumper sticker ($2.99 each, see my post #6), then make a custom bumper sticker at CafePress or Zazzle. Put the sticker on the magnet and, for less than 10 bucks (plus shipping), you've got a customized, removable magnetic bumper sticker that you can move around at will, and even paste a new bumper sticker over the top if you wish.
Plus, you can position it at eye-level to make sure the driver behind you is sure to see it. :-)
thanks.