On the world stage: Some describe the US party conventions as the greatest show on earth, but David Edgar, whose play about the presidential elections is shown next month, argues that the real drama is acted out on live TV

ONE THING that is certain about this autumn's American presidential campaign - a great deal more so than the result - is that one October evening, somewhere between 9.00 and 10.30 Eastern Standard Time, a man in a dark suit standing at a lectern in front of a blue background will turn to another man similarly configurated and call his campaign pledges into question.

Whereupon the other man will turn to the nearest available camera and say, with withering irony and in a gentle Southern drawl, 'Well, my answer to that is: 'Read my lips.' '

George Bush's celebrated (and subsequently broken) promise not to raise taxes is one of the reasons why he and Bill Clinton will lock horns on live television in October. It is almost always in the challenger's interests to debate with an incumbent, and it would be impossible for an incumbent accused of running scared to refuse. For this potential humiliation (attached to the even more delicious prospect of the goring of Dan Quayle) George Bush has to thank his two Republican predecessors.

From 1960 (when John Kennedy was seen to have trounced Vice-President Richard Nixon) until 1976 (when post- Watergate President Gerald Ford felt it prudent to debate Jimmy Carter) successive Presidents of both parties felt able to refuse the potentially poisoned chalice. From 1976 onwards, however, the debate has become a key part of the autumnal calendar. And, indeed, positively or negatively, the debates have had a significant effect on the outcome of three of the last four presidential campaigns.

They have, however, contributed in a particular way. Presidential debates are like World Cup football matches, with long stretches of purely defensive play interrupted by the occasional devastating strike. Thus no one remembers anything about the Ford-Carter debates except for Ford's assertion that 'there is no Soviet domination of Eastern Europe' (almost certainly part of a prepared answer to the wrong question). In 1980 Reagan's devastatingly folksy 'There you go again' effectively disarmed accusations that he was the dark thing from the primeval slime (contrasting with Carter's embarrassing attempt to drag in daughter Amy and her fears about nuclear war). While in 1984, after he had babbled and burbled his way through his first debate with Walter Mondale, Reagan defused the age question in debate two by assuring his challenger that he would not make political capital out of his youth and inexperience.

It's possible to argue, however, that nothing at all memorable happened in the three hours of Bush-Dukakis, with the possible exceptions of Dukakis's overwrought attempt to appear passionate in debate one ('resenting' Bush's questioning of his patriotism) and his over-adult response (in the second debate) to a question about capital punishment.

Indeed, the only really significant debating moment in 1988 was in the vice- presidential debate, when Lloyd Bentsen berated Senator Quayle for appearing to compare himself to President Kennedy.

In fact, Quayle had hardly done so, any more than then Vice-President Bush had actually patronised Geraldine Ferraro four years before. Bentsen's devastating sound-bite was the result of careful rehearsal, just as Reagan's near-disaster in the first Mondale debate was the result of a crucial debate-preparation error.

Since 1976, the process of debate preparation has evolved - along with so much else in American politics - into a matter of considerable technical sophistication. Some - indeed much - of it is a matter of working out answers to likely questions (the experts reckon they can predict 80 per cent). But ever since Nixon's uneasy gait and sweaty upper lip supposedly lost him the 1960 race (the lore is that radio listeners eventually gave him the Presidency), minders have been just as aware of the importance of the visuals.

So, as the leaves fall this autumn, lecterns will be set up in offices, dining- rooms and even garages, and candidates reminded that the average viewer sits eight feet from the screen, that they should answer the first part of the question to the questioner and then turn to camera for the rest, and that (pace Nixon) they should at all costs avoid gripping the lectern and shifting from foot to foot. They will be advised to answer hard questions short and easy questions long, not to go back over previous questions, never to gloat, and always to remember 'the bounce' (if you need to turn your head, don't do so directly, but go down to your notes, across, and then up; this is one of many good reasons for appearing to take notes even if you're not).

Although cynics argue that today's carefully structured debates are more like simultaneous press conferences, the most important part of debate preparation involves the opponent, and there is now a small and select band of men (and one woman) who are spending the late summer months boning up on, and then giving their best shot to, opinions they despise.

Thus, Robert B Barnett (a Washington lawyer of Democratic sympathies) has played George Bush in debate rehearsals for both Geraldine Ferraro and Michael Dukakis. He does it for love (as did Congresswoman Lynn Martin, who played Ferraro for Bush), but effective surrogacy can have more substantial rewards: both David Stockman (who played John Anderson, Jimmy Carter and Walter Mondale for Reagan) and Richard Darman (who played Dukakis for Bush) ended up in the same government post, as Director of the Office of Management and the Budget (equivalent to the British Chief Secretary to the Treasury), in the subsequent administrations.

Stockman is perhaps the most interesting of all, as he was advocating opinions he once held and, on one occasion, 'playing' the person from whom he learnt them. The early chapters of his autobiography map Stockman's progression from ideology to ideology (and guru to guru): coming from a strict German Protestant background, he was an anti-Vietnam war student radical in the Sixties, before becoming converted to monetarism in the late Seventies and winning a Congressional seat for the Republicans. En route between these two positions, he worked both for the maverick Democrat Senator Pat Moynihan and then liberal Republican John Anderson, who ran against Reagan for the 1980 nomination. So, in playing Anderson, Stockman was both representing and betraying the views of an old mentor in aid of a new one.

Having contributed to Reagan's victories over Anderson and then Carter, Stockman very nearly destroyed Reagan's chances against Mondale in 1984. In the first simulation (there are usually two), the task of the surrogate opponent is to win, not least in order to convince the candidate that the act needs work. On the campaign trail, Mondale had been aggressive, mean and sarcastic and so Stockman effectively destroyed Reagan in the first rehearsal ('I can hear the cement cracking under your feet,' Stockman sneered at one point; 'Oh, shut up,' was all the President could reply).

Not only was the simulation pretty bad for Reagan's morale, it was based on incorrect intelligence. The Democrats had realised that far from coming out fighting, Mondale should handle the President with kid gloves (Reagan not as Vlad the Impaler but 'the sweet old guy who can't run the family business any more').

So, come the actual debate, Reagan came up against a quite different opponent from the one he was expecting: a gently smiling Mondale who prefaced his remarks by acknowledging the respect he had not only for the presidency but also for its present occupant. Not surprisingly, Reagan was reduced to stumbling incoherence, and - for the first time - his age became a real election issue. It was not until the second debate that, re-prepped and re-energised, he was able to defuse the age issue and turn the tables.

Reagan's preparation for the first Mondale debate fell apart through getting it wrong; Bush's preparation for his debate with Geraldine Ferraro got it, if anything, too right. Faced with the delicate problem of taking on a woman, Bush's minders had decided on a 'control' strategy (the brief was not to win, but to avoid losing). Unfortunately, Bush aides had been so accurate in their prediction of how Ferraro would respond that Bush became over- confident and started trying to win. As a consequence, the pitch of his voice rose, his syntax turned to spaghetti and he gave Ferraro the inch of window she needed to accuse him of patronising her. It may well be that, even now, Clinton aides are re- viewing Bush-Ferraro to work out how to provoke the same effect eight years on.

The presidential debate is appealing as a spectator sport because, as American politics becomes increasingly bound by poll-led technology, it is an arena in which things can go unexpectedly wrong but also unexpectedly and brilliantly right.

In 1980, Ronald Reagan's campaign aides knew that their candidate had one debate objective: to convince the viewing millions they had no reason to be scared of the prospect of his presidency. So, just before Reagan walked out to confront Jimmy Carter, Reagan's campaign chief, Jim Baker, handed him a piece of paper on which was written a single-word instruction. It was 'Chuckle'.

David Edgar's 'Buying a Landslide' will be shown on 2 September as part of BBC 2's ScreenPlay season.