CNN and Fox News Channel trade barbs and talking heads.

It was the zipper heard round the world.

Over the weekend of Jan. 4, CNN ran — then quickly pulled — a controversial promo for American Morning With Paula Zahn. Over close-ups of Zahn, a narrator asks: ”Where can you find a morning news anchor who’s provocative, supersmart, oh yeah, and just a little sexy?” Then comes the much-contested noise, which CNN archrival Fox News Channel likens to a zipper, but which CNN (owned by EW parent company AOL Time Warner) claims is a scratching-record sound effect.

CNN chairman Walter Isaacson released a statement Jan. 7 calling the spot ”a major blunder” and claiming it wasn’t seen by anyone outside the promotions department before it aired. But the damage was already done. Over at Fox News, anchor Brit Hume offered a mock apology that his newscast was ”not even a little bit sexy” and included — as a coup de grace — the ”zipper” sound.

Even in the middle of an actual war, the battle between CNN and Fox News for domestic cable news supremacy has become increasingly personal. Consider CNN’s September hiring of Zahn (whom Fox fired after getting wind of the courtship) and Fox’s Jan. 2 nabbing of 10-year CNN vet Greta Van Susteren — a move widely seen as retaliation. (Fox will pay Van Susteren about $900,000, just beating CNN’s counteroffer in cash compensation.) Then there’s the spat that erupted after CNN’s Aaron Brown spotlighted a war-reporting gaffe by Fox’s Geraldo Rivera on the air. (Fox News declined to comment, saying ”We’d rather not extend Aaron Brown’s 15 minutes of fame.”)

And it’s not just the troops who are hurling brickbats. Fox News president Roger Ailes, who once compared Zahn’s ratings-getting ability to that of ”a dead raccoon,” recently predicted that Fox News would soon top CNN in ad revenue. In response, Turner Broadcasting chairman/CEO Jamie Kellner (who oversees CNN) told The New York Times: ”I’m sure [Ailes] also believes he’s better-looking than Tom Cruise.”

All the verbal grenades come at a tense time in cable news. The 21-year-old CNN maintains an edge with both a larger viewership and higher ad revenues (it outstrips Fox News in the latter by either $144 million or $23 million, depending on whether you believe Competitive Media Research or Nielsen Media Research, which gets its figures from the nets themselves). But CNN faces stiffening competition from 5-year-old Fox News, where Bill O’Reilly’s The O’Reilly Factor draws an estimated 20 million viewers per week — the highest of any show on either network. ”Fox News carved out its niche with personality programming. [Sean] Hannity! O’Reilly! Zahn!” notes Matthew Felling of the Center for Media and Public Affairs. ”Instead of focusing on news, they were focusing on people. So CNN retaliated with Aaron! Greta! Larry [King]!”

Of course, one problem with valued personalities is that they know they’re valued — and the climate of free agency leads to defections like the Zahn-Van Susteren swap. ”I wouldn’t be surprised to see a few more people jump ship in what has become a pretty lucrative bidding war,” says Washington Post media critic (and host of CNN’s Reliable Sources) Howard Kurtz. ”Picking off the other guy’s star is often easier than developing your own.”