While liberal bias is a real phenomenon – adherents of any ideology tend to be biased towards that ideology – most uses of the phrase are not based on actual evidence of bias and are primarily a form of soldier argument utilizing the argument from subjectivity – a politically-labelled version of the "That's your bias" argument, and a way to avoid losing a debate against conclusions that happen to support liberal policy without actually countering them.
Appeal to Moderation
To some extent, this may also be an appeal to the fallacy of moderation.
Anti-liberal counterfactualists see "truth" as a contest between power blocs: whoever wins gets to define it. The closest they get to objectivity is a sort of implicit admission that some people disagree with them, but they take this to mean that in order to be "objective" you must take the middle position (a.k.a. "both sides are valid", "everyone's entitled to an opinion", "there are two sides to every coin", "where there's smoke there's fire") in any contest of beliefs. If the evidence leads you to a position that is neither theirs nor one that is safely middle-of-the-road, you're choosing sides – demonstrating "bias" against that supposedly-objective middle position. (Of course, they don't apply the same standards to their own beliefs, but that's just how counterfactualism works.)
In the United States during the neoconservative era, "liberalism"/"progressivism" became largely a rationalist battle against the counterfactualism of various other ideologies – which only became more counterfactual as rationalists (defending the liberal/progressive banner) shored up their defenses and became more aggressive on their counterattacks. Attacks on liberalism tended, therefore, to be overwhelmingly counterfactual – even when there were legitimate, factual criticisms that could have been made – and rooted in perception-based thinking and cliquian signalling. Consequently, what is generally intended by an accusation of "liberal bias" in the rhetoric of this era is "you're going off-script" – you're not staying within the narrative the counterfactualists are trying to establish.
This also means that:
- If you try to interrogate their position in order to better understand its internal structure or clarify its relationship to other things, you're implicitly disputing the idea that the dominant narrative has authority over the truth. (This tends to make them angry, although they generally don't seem to understand why.)
- If you don't promptly concede to their narrative (after having been informed of your error), then you're more or less declaring yourself an enemy.
- An accusation of liberal bias is implicit in the more specific phrase "liberal media bias".
- "That's your bias."
- Wikipedia redirects to media bias as of 2015-01-25
- Conservapedia: "partisan selection or distortion of information to support liberal policies. This bias can be expressed by professors and public school teachers, College Board exams, reporters and other journalists in mainstream media, and any other information source."