Sen. John Edwards squinted into the camera in that earnest way of his and thanked Gwen Ifill for moderating, thanked the people at Cleveland's Case Western Reserve University for their hospitality and the citizens of Ohio for hosting Tuesday night's vice presidential debate. Then in his gentle drawl, Edwards extended this pleasantry to his opponent, Vice President Cheney:

"Mr. Vice President," Edwards said, "you are still not being straight with the American people."

Cheney, who was rubbing his hands together as Edwards addressed him, looked as if he was about to grab the North Carolina senator by his gorgeous hair and swing him around the stage.

And so began the 90-minute sneerfest, variously dubbed a "ruckus at the roundtable" (USA Today), "a strikingly personal and bitter debate" (the New York Times) and a donnybrook that brought the 2004 presidential campaign "to a new level of acrimony" (the Boston Globe). It is part of a continuum of presidential debates -- and more often vice presidential debates -- where the rhetoric veers into the undefined zone between "spirited exchange of ideas" and "personal attacks." Some call it "the line," as in the line of decorum that debate participants should not cross, at risk of being deemed "mean-spirited" or, literally, "over the line."

In many cases, staying this side of "the line" simply means using code words in lieu of more charged rhetoric. At last week's presidential debate in Coral Gables, Fla., for instance, moderator Jim Lehrer said that John Kerry had accused President Bush of "lying to the American people about Iraq."

Kerry corrected Lehrer, saying "I've never used the harshest word, as you did just then, and I try not to." Rather, Kerry said, Bush "has not been candid with the American people."

But he wouldn't call the president of the United States a liar. That would be over the line.

Like art and porn, what constitutes a breach of "the line" rests in the eye of the observer. Neither Cheney nor Edwards was accused of crossing the line Tuesday, but debate watchers say their contretemps were particularly harsh nonetheless.

"Definitely one of the sharpest debates I've seen," says Alan Schroeder, author of "Presidential Debates: 40 Years of High-Risk TV." He says Cheney-Edwards nearly approached the nastiness of a previous benchmark -- the 1976 debate between Walter Mondale and Robert Dole, in which Dole bemoaned all the U.S. servicemen killed in "Democrat wars" through history.

While Edwards-Cheney is unlikely to be recalled for a single line, Schroeder says the mutual enmity was palpable. When Cheney mocked Edwards for his poor Senate attendance record by saying that he had never met Edwards before Tuesday night, he did it in a way "that went well beyond a critique of his showing up to vote," Schroeder says. "That was Cheney's way of saying, 'I know everybody in this town and who the hell are you?' "

Schroeder was struck by the lack of any pleasant preamble or soft windup by Edwards. "He just came out of the gate swinging and it was on," he says.

This is how it should be, says Alan Simpson, the former Republican senator from Wyoming, who attended Tuesday's debate.

"Heck, we're not playing powderpuff bowl here," says Simpson, a close friend of Cheney. "Heck, my old man ran for Senate in 1940 and they burned his car." In other words, to heck with the "line."

In Cleveland, Simpson sat a few yards away from Sen. Patrick Leahy, the Vermont Democrat with whom Cheney famously crossed "the line" last spring when the vice president told Leahy on the Senate floor to "go [expletive] yourself."

"If Cheney had tried that with Edwards, he'd have gotten in trouble with the FCC," says Marshall Wittman, a senior fellow at the Democratic Leadership Council who until recently was press secretary for Sen. John McCain (R-Ariz.).

In general, Wittman says, a politician can get away with slandering an opponent if he addresses the recipient as "my distinguished colleague" or "the distinguished gentleman from wherever."

Simpson often waxes nostalgic about Republican Sen. Wayne Morse of Oregon, who, according to a 1957 newspaper account, called his "distinguished colleague" Republican Sen. Homer Capehart of Indiana "a tub of rancid ignorance."

Those were better days.

Sen. John Edwards set the tone of Tuesday's debate early, wasting no time in going after Vice President Cheney.