WASHINGTON // There is a moment in almost every US presidential debate that comes to define it. In 2000, there was Al Gore rolling his eyes and sighing in exasperation; in 1992, George H W Bush was caught glancing at his wristwatch; in 1976, Gerald Ford mistakenly declared - twice - there was no Soviet domination of Eastern Europe. Tomorrow is the first of three televised debates between John McCain and Barack Obama, although Mr McCain late last night called for a postponement of that debate, citing the need to focus on the financial crisis. Mr Obama, however, ruled out delaying the debate, and also rejected joining Mr McCain in suspending his campaign, saying both candidates had private jets that could easily ferry them from Mississippi to Washington if they were needed. Both the University of Mississippi - where the debate is being held - and the debate commission said there had been no formal request for a postponement and were going ahead with plans to host tomorrow's event. When the debate does get under way, each of the candidates will be trying to talk himself up, bring his rival down and avoid the kind of memorable moment that can turn voters off and even potentially swing an election as close as this one. The presidential debates - the second most watched event on US television after the Super Bowl - typically reinforce voters' impressions rather than change them, meaning few who go in leaning towards Mr McCain are likely to come away favouring Mr Obama. But the debates provide the candidates one of their last, best chances to appeal to a critical segment of the electorate in the campaign's home stretch: those who have not yet made up their minds. "It is the only opportunity that the American public has during the entire campaign to see both candidates on the same stage at the same time answering basically the same questions and directly responding to one another, instead of doing it in a delayed fashion through 30-second spots," said Diana B Carlin, a professor of communication research at the University of Kansas who served for more than a decade on the advisory board of the Commission on Presidential Debates. "The public views these as a job interview," she said. These debates - tomorrow's on foreign policy, the second on issues chosen by voters in a "town meeting" and the last on domestic matters - could draw record numbers of viewers. Forty million people tuned in to each of the candidates' nomination acceptance speeches at the political conventions, and debate viewership could be double that. Mr Obama had been holed up in Florida this week at "debate camp". He has held mock sessions opposite Greg Craig, a former Clinton administration official and lawyer standing in for Mr McCain. Mr McCain, meanwhile, had been practising against Michael Steele, the former lieutenant governor of Maryland. No detail, from when to look in the camera to what body language should be avoided, is too small. Part of every candidate's strategy for debate success is managing expectations, especially in a media climate in which pundits will almost immediately declare a "winner" (even if there is not a clear one). Previous contests offer some indication of where each candidate may excel or come up short. Mr Obama, who performed unevenly during the more than 20 Democratic primary debates, is known for delivering inspirational speeches in front of large audiences. But that is with a Teleprompter, a script and time to build to a rhetorical climax. The debates, by contrast, reward crisper, pithier answers and, despite the huge television audience, put the candidate in a more intimate setting - one where Mr Obama has seemed to have trouble connecting with voters. Although the Illinois senator almost always remains cool under fire, those who study debates say he must avoid sounding like the lawyer that he is, and coming across so cool as to seem passionless or aloof. Mr McCain, while not terribly gifted at giving a stump speech, is much better at boiling his message down to a well-articulated sound bite; as such, he had some applause lines in this year's Republican debates. He is an aggressive debater but can display an affability and sense of humour that voters appreciate. But observers said the Arizona Republican must guard against showing any aspect of his famed temper, as he did with some of his primary opponents. Even despite the hours candidates invest in preparation, the debates are one of the few moments of the campaign that are relatively unscripted and where unexpected things can and do happen. And that, said David Birdsell, co-author of Presidential Debates: The Challenge of Creating an Informed Electorate, is why they often tell voters so much. "You get to see people actually thinking on their feet, responding to their preparation, but also responding to the pitch and tilt of the questions and the behaviour of their opponents," said Mr Birdsell, dean of the School of Public Affairs at Baruch College at the City University of New York. The first televised debate, between John F Kennedy and Richard Nixon in 1960, was a prime example of the power of image: Mr Kennedy came across as comfortable and telegenic, while Mr Nixon, who declined to wear make-up and was sick with the flu, appeared shifty and ill at ease. According to one study, TV viewers thought Mr Kennedy won the debate, while those who listened on the radio thought Mr Nixon did better. "From that point, it became as important to look good saying things in addition to just saying the things," said Bruce DuMont, president of the Museum of Broadcast Communications in Chicago and host of Beyond the Beltway, a nationally syndicated radio show. "I would say a significant portion is cosmetics and style." Good debaters, of course, do everything well. They know the issues inside and out, but do not sound scripted. They seem comfortable in front of the camera without seeming to play to it. They can deftly criticise their opponent, but avoid coming across as nasty. In 1984, when Ronald Reagan was asked whether age - the president was 73 - would be an issue, he drew hearty laughter, including from Walter Mondale, his Democratic opponent, when he replied: "I will not make age an issue in this campaign. I am not going to exploit, for political purposes, my opponent's youth and inexperience." "Debates are educational, certainly, but maybe they're educational in more of a visceral way," said Alan Schroeder, associate professor of journalism at Boston's Northeastern University, who wrote Presidential Debates: 50 Years of High-Risk TV. "It's more of judging the candidates as human beings." eniedowski@thenational.ae