Americans are more than twice as likely to say media coverage of Obama is unfairly positive as to say it is unfairly negative. For McCain, the opposite is true, with many more seeing coverage of him as unfairly negative than as unfairly positive.
This is as absurd as believing that grizzly bears are harmless fuzzy friends who like to be petted, or that red hair is a sign of moral depravity. It is extremely well documented that the media have avoided challenging McCain's false statements or even cut his most preposterous gaffes out of interviews. There is no way to characterize the belief that the media are unfairly negative in their coverage of McCain as anything other than delusional.
But why are Americans so stupid? Why do they so readily embrace this ludicrous myth of the liberally-biased media? How is it possible for those who spread the lies to convince most people all the time? (Note that the belief is not limited to Republicans or conservatives or McCain supporters: a fifth of intended Obama voters also believe that the media coverage of McCain has been unfairly negative.)
I think it's the fault of those goddamn liberally-biased facts. For decades, the Republican candidates have consistently been so much dumber, meaner, and more dishonest than their Democratic counterparts that it is hard for most people to believe that such pervasive differences are real. Surely, God created the two parties equal, so the vast difference in quality must be an illusion created by the media.
This has an interesting corollary: the best way for the Right to perpetuate the myth is to support incompetent, cranky, and mendacious candidates. And if that's part of the plan, then all I can say is, they are getting more and more successful!
UPDATE: I just found this Daily Kos diary, which discusses the same issue and shows every step of the reasoning - user-friendly for the pedestrians.