The incriminating word had long been a land mine because of its potential to damage the presidential candidate making the accusation. In 1988, Senator Bob Dole, running for the Republican nomination, appeared on an NBC interview with his rival, Vice President George Bush, and snapped at him, “Stop lying about my record.” It cemented an image of Mr. Dole as a meanie  which he had to wrestle with as the party’s nominee in 1996, when, for instance, some television viewers bristled as he accused Katie Couric of carrying water for the Democrats.

Neither Ms. Couric nor Tom Brokaw, the NBC interviewer in 1988, tried to be an arbiter in those moments, leaving viewers to make up their own minds about Mr. Dole. Mr. Kerry, meanwhile, hoped that the news media would judge a lie as a lie when he was battered by the Swift Boat Veterans for Truth over his military record in Vietnam, but the media only did so much.

“When politicians ignore a lie and it doesn’t go away, they can be tainted, and when they deny it, they can sound defensive,” said Geoff Stone, a University of Chicago law professor and an informal adviser to Mr. Obama. “Americans need someone who many people watch who has credibility and can be an independent arbiter  a Walter Cronkite, say  and there isn’t one.”

Mr. Kerry’s hesitancy may have been related to his constitutional disposition for Marquess of Queensbury rules; the prep-school-bred, Yale-educated Brahmin was never comfortable with slashing attacks. But mostly his concern was strategic: Mr. Bush enjoyed approval ratings of 50 percent or more in the fall of 2004, and he was often ahead in the polls of a closely divided electorate.

“We wanted to force a general election vote around the issues, especially domestic issues, and calling Bush a liar would have taken us off in a different direction,” said Bob Shrum, Mr. Kerry’s chief strategist.

Some Obama advisers were gun-shy early on about calling out an alleged lie  especially against Senator Hillary Rodham Clinton  but they snapped to it with Mr. McCain. Mr. Obama himself, who has often spoken of elevating the political debate, still acts a little hesitant in his accusations: in Nevada last week, he would only say that a variety of McCain ads were “patently wrong.”

Whether he drops the L bomb in the televised debates over the next three weeks is now a point of discussion within the Obama campaign, aides say. Saying that word in front of tens of millions of viewers could be a very risky proposition for the candidate running on a platform of hope. No lie.