It takes only an instant for a visitor to Media Matters for America's headquarters in downtown Washington to sense its mission, if not its methods. A few steps into its modern offices, which resemble a newspaper newsroom, a pair of prominently displayed signs spell out the basics: "Fox Keeps Fear Alive," reads one; "Restore Sanity, Fight Fox," reads its companion.

Fighting Fox is what Media Matters does, relentlessly and obsessively. In the six years since its founding, the watchdog group has evolved from an all-purpose scourge of the conservative media into Fox News Channel's veritable shadow and constant irritant. From well before sunrise to long after it each day, teams of young researchers sift through video clips and transcripts of programs hosted by Fox stars such as Glenn Beck and Bill O'Reilly to find dubious facts, logical contradictions and poisonous - at least to Media Matters' liberal sensibilities - rhetoric.

Media Matters watches other conservative media figures - your Rush Limbaughs and Michael Savages - but Fox has become the focus. On most days, the majority of the blog items, video excerpts and commentaries that the organization posts to its Web site concern some perceived outrage perpetrated by one of Fox's pugnacious hosts and commentators.

"Sarah Palin joins Glenn Beck in desecrating MLK's legacy," read a headline on the site the other day.

"Fox News hosts tell [billionaire Warren] Buffett to 'quit lecturing' the rich," read another.

"REPORT: Fox donates at least $40 million in airtime to potential GOP presidential candidates," read a third on a posting that cited Media Matters' estimate of the value of airtime soaked up by Fox commentators/would-be presidential candidates Palin, Mike Huckabee, John Bolton, Rick Santorum and Newt Gingrich.

The dogged pursuit of Fox, says Media Matters' founder David Brock, reflects not just the cable network's popularity among conservatives but its power to set, and perhaps distort, the political agenda. Brock and his staff say they regard Fox as something more than just the televised equivalent of talk radio; they describe it as a de facto political operation, with a leading role in disseminating conservative messages, supporting conservative candidates and mobilizing voters.

"I don't consider it a media institution," Brock says. "It's a political institution that [Fox News Chief Executive Roger] Ailes created after Obama came into office. . . . We're here to counter their lies and misinformation."

Brock is a story unto himself. A former journalist-turned-political operative, he made his name in the 1990s as a self-described conservative "hit man," with scathing exposes of Anita Hill (his book was titled, "The Real Anita Hill") and of Bill Clinton and his accuser, Paula Jones.

In the latter part of the decade, however, he disavowed his conservative convictions and became a liberal. His conversion was recounted in a confessional book, "Blinded by the Right."

Fox's hosts haven't been shy about firing back at Media Matters - apparently to Media Matters' delight. The group keeps an archive of Fox's periodic attacks on the organization on the front page of its Web site. One is a clip of "Fox and Friends" morning host Steve Doocy dismissing Media Matters as "a blog nobody reads." O'Reilly, a frequent Media Matters target, has called the organization "a vile propaganda outfit." (Fox did not respond to repeated requests for comment.)

As colorful as the tit-for-tat battle is, it may be just the foreground noise of a larger war between rival billionaires. In one corner: Rupert Murdoch, the chief executive and principal shareholder of News Corp., Fox's parent company. In the other: George Soros, the wildly successful investor whose philanthropic organization, the Open Society Institute, has funded many of the organizations that Fox's personalities like to attack, including Media Matters.