The fact that it’s a bumper sticker is offensive enough. They were just making your car less tacky!
Seriously though, it depends on what’s underneath the tape: I imagine your question has more to do with removing the tape without damaging the car rather than trying to preserve the bumper sticker. If it’s on a factory-painted surface, I’d recommend soaking the tape in hot, (carwash) soapy water for several minutes. The heat should loosen it up enough to safely remove. Then you’ll just need some kind of bug/tar remover to get rid of the glue.
Warm soapy water is the best way to get the glue to release. It’s best to use a dish soap like Dawn or Joy. You can clean any residue easily with some bug and tar remover from your local autoparts or Walmart store, or plain old elbow grease if you rather work for it.
Duct tape is actually fairly gooey and not as tacky and sticky as, say, packing tape. Heat that shit up either with a day in the summer Vegas sun* or something like it then slowly peel it off; slowly like the same way you remove those stupid Proof of Purchase stickers from new XBox 360 games. This will remove most of it and the gooey remnants can be rubbed off by the palm of your hand in a rolling/rubbing motion.
Agree with the “heat it with a hair dryer and then pull it off and use Goo Gone” technique.
I don’t get why you think taking duct tape off your car might hurt the paint. If it wasn’t chipped or scratched or anything, I expect the painted surface of a car could withstand having a piece of duct tape removed from it, especially if it’s only been on a day or so.
And you put bumper stickers on the painted part of your car? Unless this is a bumper that’s the same color as the rest of the car, I’m also puzzled.
You could try some kind of solvent. You can also try WD-40. Saturate the tape with WD-40, let it soak a bit and try and peel it off. You may need to repeat the soakings.
WD-40 will dissolve the glue completely if it can get at it. You could also try and get a peel going and then spray WD-40 on the seam, wait a bit and peel it back even more. Repeat until is removed. If there are any gooey bits left, just make sure they are soaked with WD-40 and give them 20 min and then it should wipe off.
BTW where did this happen? A parking lot or when you came out for work one day your car had the duct-tape on it? I wouldn’t think people carry around duct-tape so it must have been pre-meditated, and must be someone who saw it regularly.
I’d shy away from WD-40, as WD-40 can be unfriendly to some paint and clearcoats. WD-40 has a way of hazing clearcoats if not removed quickly and completely, and can cause the clearcoat to lift off the basecoat. You won’t notice it right away, but after some time in the sun, or a few carwashes, it will start to flake away.