Here, http://boards.straightdope.com/sdmb/showthread.php?p=5439578#post5439578 , a post I started to respond to, then it turned into a thread OP.
Actually, there isn’t that much of one in reporting. There is in TV news an anti-fringe bias; & a tendency to place personality as important in choosing officials, while policy is seen as too debatable to have strong opinions on.
But where a “liberal bias” exists, it’s often not a bias toward the “loony left” but toward moderate conventional wisdom, toward being inoffensive to those in power, or just toward facts. So the fringes on both left & right see the media as biased against them, because (a) the fringe on both right & left has a seriously distorted view of political reality, & (b) the media try to be nice to everybody, including those who strongly disagree with one’s side, so this offends those who are sure they’re right.
There’s an piece in a recent Atlantic Monthly about this, as I recall.
Another aspect can be put this way: If 60% of the voting public believes that all Mexicans are agents of an alien power trying to subvert our country, or that blacks have no souls, but the best science says they’re wrong, any reporter who simply acknowledges the truth will be seen as playing to “the other side.” In actual history, Reaganomics, for example, was based on two false analyses: of where on the Laffer curve we were, & of how the rich would respond to a tax cut. It may have “increased prosperity,” but it demonstrably never produced the “rising tide to lift all boats” that it promised. But for Reaganites, tax cuts are categorically good, so to tell the truth about Reaganomics’ flaws is seen as an attack on their dogma, like telling a Catholic that he’s just eating a cracker after all. So many reporters, not being economically savvy, & chastened by angry supply-siders, will allow that maybe Reaganomics sort of worked. But the GOP side will then claim that there’s this horrible bias against “their” economics (as if science is subjective) when all that’s biased is the facts, which maybe got reported, 'cos they’re the facts.
Peter Jennings once said, as I recall, that of course you can’t really be totally impartial (at least on the inside, a reporter has his own beliefs)–but you try to be fair. Those who favor partisan reporting are kidding themselves if they think it’s going to be better than fair reporting. A schismatic media will simply turn to propaganda, & then nothing could be trusted.