Everybody thinks the media are biased against their side, and conservatives are particularly likely to believe it. They themselves would say "That's because it's true!", but the real reason is that the complaint of liberal bias is one that conservatives hear all the time from all of their media sources. That isn't to say there aren't some issues on which the conservative side doesn't get equally favorable coverage, because there may well be a few, just as there are issues on which liberals get the short end of the media stick. But on some you can make a case that there are legitimate reasons. For instance, I wouldn't be surprised if a systematic analysis revealed that coverage of the gay marriage issue was friendlier to the pro side. That might be because one side is arguing for equality and the other side is arguing for discrimination, and portraying the two as equally morally valid is itself problematic.
Anyhow, if there's ever a topic about which coverage should be emphatically even-handed, it's an electoral campaign. You've got two sides trying to achieve the same objective, both of whom represent large portions of the public. Aha, conservatives would say—but coverage of elections is totally biased against Republicans! And when you ask them to support this claim, their evidence usually comes in two forms. One is, "Here's an example of a story that was totally mean to our candidate!"—in other words, an anecdote. The other is, "If you can't see it, then you're hopeless." Which of course is no evidence at all.
But what happens when you actually try to analyze news coverage of campaigns in a systematic way? The results usually look like these, which come from John Sides and Lynn Vavreck's new book about the 2012 election, The Gamble: