Well yeah, of course it has, with all the help the conservatives' political opponents have provided.
You want examples? Look at the last presidential campaign. In the preceding eight years, W hardly got away with anything without being denied the benefit of the doubt. The Iraq war was just over its first hump when people started complaining about the lack of an exit strategy (not that it's bad to have one, but it's weird to focus on that while skipping right over things like, say, a victory strategy; maybe the armchair generals thought a successful first wave was just how wars work in the post-Vietnam era). Then a freshman congressman from Chicago, with hardly anything to his voting record, shows up; and not only is treated like the Second Coming, almost no one is heard saying "Um, that's a little beyond enthusiastic, isn't it? Can we talk about what he plans on doing instead of how he's going to usher in the Age of Aquarius? Who is lucid or sane enough for us to be quoting seriously, who goes around talking about Obama like he's the übermensch?"
And I know it's not exactly the same thing, but it's related, so while we're discussing bias, we should also talk about slant. Things where articles give equal time but one side enjoys the application of euphemisms or more favorable adjectives. Things where articles are objective in content but give more air or screen time or column space to one side than to another. Things where articles are pretty well balanced but giving one side an apparently decisive but premature last word. Things where biased experts are brought in to analyze a situation and fall short of reasonable efforts to keep his preferences out of the discussion.
But let's also talk about media magnates who visit journalism schools and tell students that not only is true objectivity impossible, leftist bias is preferable--even an obligation. Let's talk about the folks in the media who apparently missed those lectures and insist that they and theirs are capable of rising above it all. Let's talk about industry language standards that clearly favor one side, to the tune of using "pro-choice" for the side that favors abortion while using "anti-choice" for the side that considers it to be the worst of murders. Let's talk about how Fox news doesn't just seem conservative, but seems predominantly conservative against the bulk of the rest of the media.
I can only conclude that they have some radically different definition of "bias" from the rest of us. I wonder what they have in mind when they use the word. I wonder what they think we're talking about, when we use it.
But biased or not, I don't put a lot of stock in the mass media as a stand-alone source of predigested news. They're ignorant or sloppy enough at reporting on the topics that I understand well, that I'm inclined to think they perform about as well in reporting on other topics I'm less equipped to judge. That doesn't leave much worth watching except the weather and traffic. Oh, and sports, I guess.