Ezra Klein and Matthew Yglesias, respectively, have nice posts on two wildly popular misconceptions about politics:
1. Your ideological opponents are vastly better organized than your side.
2. Your side is losing/has lost because the media is biased against you.
#2 in particular is part of a more general phenomenon in which people search for and then latch on to visible explanations of political events. For instance, George H.W. Bush's win in 1988 can be seen as a convergence toward the political fundamentals, but if you don't study presidential elections, that's an abstract concept that doesn't lend itself to media-friendly narratives. So instead the media made up stories about Michael Dukakis losing because of Willie Horton, etc. In the same way, when people have a hard time understanding what's going on in politics, they tend to blame the media, which often reflects the prevailing political climate of the time. The most recent example is the way the media was faulted for being soft on President Bush in the post-9/11 period, which was almost surely the consequence of the political circumstances rather than the cause of them.
(None of this is to say that the media can't have a causal effect on political outcomes; only that it's easy to reach exaggerated conclusions about its effects by assuming correlation=causation.)
Update 7/20 7:46 PM: You can also think of #1 as reflecting people's search for explanations of causal events. It's more comforting to blame a failure of tactics or nerve or the other side's overwhelming dominance than to concede that your side was rejected by the public. (See, for example, the Wall Street Journal's implausible attempt to blame the GOP's 2006 losses on the Republicans not being conservative enough.)
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