Fox News is the subject of dozens of articles, at least one book (“Crazy Like a Fox” by Scott Collins) and now a full-length documentary (Robert Greenwald’s “Outfoxed”). Fox is revered and reviled for its political tilt, which it both embraces and disavows. Fox brandishes its “fair and balanced” slogan, paradoxically, as both a repudiation of bias and as code for the rebalancing it promises to offset the liberalism alleged of its rivals.

Archrival CNN draws more total viewers per day, as people check in for news updates. But because Fox’s audience tends to stay put, treating it like talk-radio, it now has more people tuned in at any given moment; hence its plausible claim to have overtaken CNN as cable news leader.

Not bad for an outfit that’s not yet a decade old. Its real history began only in 1996, when the former GOP strategist Roger Ailes defected from MSNBC, the hapless Microsoft-NBC partnership. Ailes took over the Fox news division that Rupert Murdoch targeted in response to surveys that found that sizable numbers of people believed U.S. media to have a leftist bias.

Still, Fox is relatively puny. Its highest-rated show, “The O’Reilly Factor,” pulls 2 million viewers. By contrast, as Steve Rendall noted in an online analysis, the “CBS Evening News” gets 8 million to 10 million — considered a ratings disaster.

But Fox’s influence far exceeds its reach. Fox News poses an outsized challenge.

With the imperatives of cable favoring media that target and deliver audience slivers of predictable composition, Fox is pioneering the use of political ideology to create and sustain a market. Others are learning.

I’d never really watched Fox, so I spent a week tuning in. I expected a combative, biased, factually corrupt report. Instead, by day I found a professional news operation.

There were limits. The staff seemed small. As usual with U.S. media, news from abroad was sparse. Fox did carry the GOP’s water on some stories, seemed to deliberately downplay Iraq and ignored the John Kerry campaign.

But the guests were more diverse than on rival networks, where U.S. experts generally speak for the entire human race; anchors were sharp and well-briefed, and they baited conservative guests from the left and liberals from the right. The report was well-paced, insightful and engaging.

Then night fell, the trunk lid creaked open, and the bats flew out. Sean Hannity was simply a bully, without finesse or subtlety. When the subject was trial lawyers, he asserted, essentially, that Sen. John Edwards had halted innovation throughout the economy and kept millions of Americans from receiving health care.

But Bill O’Reilly was a revelation, smug and derisive. In a signature moment with ex-House Speaker Newt Gingrich, they nodded sagely over a claim that 80 percent of the country shares core values, leaving only media and university liberals — Hollywood, too, O’Reilly said — outside the “American” consensus.

Before they broke for a Bush re-election ad and the Laci Peterson murder trial, the two were half-step away from redefining citizenship as excluding Democrats.

In Scott Collins’ insightful book, O’Reilly says that his model was a newspaper’s opinion pages. But the difference is huge, and that’s what makes Fox so unsettling. A typical newspaper has commercial reasons for trying to widen its audience and therefore appeals to an expansive notion of common interest.

A niche news channel has a different commercial model. It seeks to attract and continually reclaim an audience fragment, by reiterating the worldview that first drew them.

It’s a model that builds a corps of the faithful but gains nothing from persuasion, let alone the give-and-take by which a political culture thrives. Fox is pioneering a national conversation in which we have nothing to offer one another but reassurance — or abuse.

Edward Wasserman is Knight professor of journalism ethics at Washington and Lee University. He wrote this column for The Miami Herald. E-mail him at edward–wasserman@hotmail.com.