I advise refraining from using bumper stickers.
IMO, nobody is convinced by the messages they contain, and they just make the driver feel good.
I used to have some on my van, until my back window was broken by someone who didn't like the stickers.
Then you have idiots who key cars or think that they have the right to remove the stickers themselves. It's just not worth the trouble.
A bumper sticker isn't an argument, it's a cheer.
It's meant to make everyone else who feels the same way feel good, and lets them know they're not alone.
I take it you believe all campaign signage is useless as well?
Ironic screenname you have there.