This, from Hendrik Hertzberg's Comment in the current New Yorker, seems exactly right to me:
... lunatic paranoia—touched with populism, nativism, racism, and anti-intellectualism—has long been a feature of the fringe, especially during times of economic bewilderment. What is different now is the evolution of a new political organism, with paranoia as its animating principle. The town-meeting shouters may be the organism’s hands and feet, but its heart—also, Heaven help us, its brain—is a “conservative” media alliance built around talk radio and cable television, especially Fox News. The protesters do not look to politicians for leadership. They look to niche media figures like Rush Limbaugh, Sean Hannity, Glenn Beck, Michael Savage, and their scores of clones behind local and national microphones. Because these figures have no responsibilities, they cannot disappoint. Their sneers may be false and hateful—they all routinely liken the President and the “Democrat Party” to murderous totalitarians—but they are employed by large, nominally respectable corporations and supported by national advertisers, lending them a considerable measure of institutional prestige. The dominant wing of the Republican Party is increasingly an appendage of the organism—the tail, you might say, though it seems to wag more often from fear than from happiness.