The bland even-handedness of network news anchors and late-night yucksters may be increasingly out-of-date, eclipsed by the partisan antics of the hosts of Fox News and MSNBC. But the middle-of-the-road personalities who've dominated network television since its inception do retain one singular power: when they finally get off the fence, the mass of similar-minded voters take notice.

That's why Edward Murrow—famed for his measured wartime broadcasts from London—had such an effect when he took on red-baiting Senator McCarthy. (Here's a scene from Good Night and Good Luck, a movie account of that confrontation.) And it's why Late Show host David Letterman's disillusionment with Republican candidate John McCain, evidenced two nights in a row now, could swing the presidential campaign. "The road to the White House runs through me," joked Letterman on Wednesday night when he berated McCain for suspending his campaign and canceling a planned appearance on the CBS late-night show.

One can attribute Letterman's ridicule of the 72-year-old Republican to personal pique; McCain appeared on Katie Couric's CBS news show when he was supposed to record with Letterman. But it can't be dismissed as kneejerk partisanship. Indiana-born Letterman was broadly in favor of the military response after September 11th and even when turning on McCain he's been at pains to describe the war veteran as an "honest-go-God American hero."

If the late-show host is off the fence, that's a powerful signal to news reporters who haven't been sure how to spin—cynical or brilliant?—the McCain campaign's latest stunt. No one media personality can puncture a politician as Murrow once could. But Letterman can influence more media and public opinion than all those preachers-to-the-converted such as Sean Hannity and Keith Olbermann on the cable news shows.