The only reason I'd put a bumper sticker on my car would be for something that I strongly believe. This belief could be political in nature, for instance, free education at all levels. I believe that if we are all equal, we also need to have the same opportunities and education is one of those.
However, I don't share ALL the positions of any particular political party. For instance, a political party that bost free education may also promote hatred in the world by being offended by everything or being utterly blind to the dangers of religion in society.
Hence, I don't see myself putting a sticker that promotes a specific party.