The short version: People with bumper stickers of all political bents seem to drive more aggressively.
Related: Bob Sutton describes a 1975 study that used bumper stickers, along with gun racks, as stimuli:
“[Researchers] manipulated the situation so that a pick-up truck at a stop light was slow to start moving after the light turned green. They measured aggression by how quickly and how intensely the driver behind the truck started honking. Turner and his colleagues varied two things about the pick-up truck: a gun rack with or without a gun, and two different bumper stickers. One said “friend” and the other said “vengeance.” It is an interesting study because many people — including me — predict in advance that the gun and vengeance stickers would lead to do less honking, as the impatient driver waiting behind the truck might fear getting shot by the aggressive and armed person. In fact, Turner and his colleagues found the opposite pattern. The drivers stuck behind the truck were more likely to honk when the driver had a gun, and even more likely to honk when he had both a gun and a vengeance bumper sticker! One explanation is that aggression breeds aggression.”