Having quoted Julian Sanchez earlier, let me quote another excerpt from a very smart post of his. Sanchez argues that conservatives are so determined to discredit internal dissidents because those dissidents are especially dangerous to their system of epistemic closure:
One of the more striking features of the contemporary conservative movement is the extent to which it has been moving toward epistemic closure. Reality is defined by a multimedia array of interconnected and cross promoting conservative blogs, radio programs, magazines, and of course, Fox News. Whatever conflicts with that reality can be dismissed out of hand because it comes from the liberal media, and is therefore ipso facto not to be trusted. (How do you know they’re liberal? Well, they disagree with the conservative media!) This epistemic closure can be a source of solidarity and energy, but it also renders the conservative media ecosystem fragile. Think of the complete panic China’s rulers feel about any breaks in their Internet firewall: The more successfully external sources of information have been excluded to date, the more unpredictable the effects of a breach become. Internal criticism is then especially problematic, because it threatens the hermetic seal. It’s not just that any particular criticism might have to be taken seriously coming from a fellow conservative. Rather, it’s that anything that breaks down the tacit equivalence between “critic of conservatives and “wicked liberal smear artist” undermines the effectiveness of the entire information filter. If disagreement is not in itself evidence of malign intent or moral degeneracy, people start feeling an obligation to engage it sincerely—maybe even when it comes from the New York Times. And there is nothing more potentially fatal to the momentum of an insurgency fueled by anger than a conversation.
To some extent, you can find an equivalent to this on the left, where there are innumerable attacks upon pseudo-liberals who discredit true progressives in order to enjoy the polite company of David Broder. The left has created its own counter-network of ideologically sympathetic opinion outlets on blogs, Rachel Maddow and Keith Olbermann, and so on -- places where liberals can find constant confirmation of their own worldview.
The difference is that liberals do not see these outlets as replacements for the news. In the conservative worldview, mainstream media is not just flawed but fatally tainted by deep ideological hostility. Millions of conservatives believe the only sources of credible news are Fox News, Rush Limbaugh and the like -- even a figure like Clarence Thomas once told an interviewer that his sole sources of news are Limbaugh and the American Spectator. Liberals may seek out ideologically friendly sources to augment their information intake, but the phenomenon of total epistemic closure that Sanchez describes is almost entirely limited to the right.