I love the urban legends site Snopes.com, both for reference, and just for browing around. Following up on the Willisms post on false quotes by liberals - it has an extended and fairly pointless discussion in the comments, but some of you will like it well enough because of subject matter - I browsed snopes for what quotes and actions had been attributed to George W. Bush. The negative rumors were mostly false, the positive ones mostly true. On a hunch, I tried the same trick for John Kerry, Al Gore, Bush 41, Dan Quayle, and Bill Clinton. Same political pattern, in that there were many negative rumors about Republicans which later proved false, many negative rumors about Democrats that later proved true. For positive rumors, the reverse.
It wasn't anything like 100% - 0%, or so overwhelming that one would suspect the evidence was just too tidy to be real. Just a pretty strong tendency. Check it out yourself, if you like.
I had previously heard less than half the rumors either way, so I can't tell you how generally believed these urban legends were about the various political figures. To come to the attention of snopes, some threshhold of general circulation must have to be reached. This would suggest that people more readily believe negative myths about Republicans, but it is only evidence for that theory, not proof of it.