True to his image as the wisecracking gunslinger, Ronald Reagan was wearing a plaid shirt and cowboy boots when he walked on to the stage to debate President Jimmy Carter. It was October 1980. With the polls deadlocked and just a week to go before the American people voted, the rhetorical confrontation between the two leading candidates for the presidency was important. Confident as ever, Reagan sauntered out beneath the spotlights with a cheerful grin. And then Carter went for him. He read out old Reagan quotations in a mocking tone; he reacted to Reagan's answers by shaking his head and saying, scornfully, "That's the most ridiculous thing I ever heard"; he pointed accusingly after one reply and said: "This just demonstrates his shallowness."

Meanwhile, the so-called Great Communicator seemed all over the place. His performance was "miserable", his opponent later recalled. "I was shocked. He couldn't fill up the time. His answers weren't long enough. And what time he could fill, he filled with woolly platitudes."

It was lucky for Reagan that nobody saw his lamentable performance, but then they were never going to. It was a rehearsal, not the real thing: the stage had been set up under spotlights in a garage in Virginia, and his "opponent" was the Republican congressman David Stockman, who had prepared using a stolen copy of the Democrats' briefing book. He played Jimmy Carter better than the president did himself. Afterwards, Stockman drove home in a mood of silent despair, convinced that Reagan, with no time for another rehearsal, was about to throw away the election on television.

Now that Britain's political leaders have decided to follow their US counterparts by agreeing to take part in a series of three televised debates to be held on 15, 22 and 29 April, it is a safe bet that Gordon Brown, David Cameron and Nick Clegg will have seen more than a few clips of Reagan's performance a few days later. Like him, they will have spent hours learning their lines, working on their put-downs, honing their impersonations of seriousness, sincerity and compassion. Like him, they will have made more than a few howlers in rehearsal; apparently, Brown has been practising against Alastair Campbell, while in Cameron's rehearsals, Michael Gove plays Brown and Jeremy Hunt plays Clegg. Above all, though, they should have learned the single biggest lesson from the history of televised debates: what matters is not substance, but style.

Dangerous territory

From the moment Reagan and Carter walked out beneath the lights in Cleveland, Ohio, it was clear that the difference between them was as much physical as ideological. Drained after four years in the Oval Office, Carter looked stiff and nervous, his eyes puffy, his face taut with tension. Reagan looked younger than his 69 years (a reminder that every candidate needs a good make-up artist): tall, calm, determined. Before they started, he strode across to Carter and held out his hand - a gesture that took the president by surprise, and brought home the difference in stature and confidence between the two.

Like most such encounters, Reagan-Carter was less a genuine debate than a chance for the candidates to speak their prepared lines and to remind the viewers of their most popular positions. As Brown's, Cameron's and Clegg's handlers should have told them, the trick is to work with viewers' preconceptions, deftly neutralising sensitive issues and steering the conversation on to familiar territory.

In 1980, Reagan's very first answer - "I'm only here to tell you that I believe with all my heart that our first priority must be world peace" - was designed to puncture Democratic claims that he was a warmonger. He used the word "peace" more than almost any other, and when Carter accused him of reckless belligerence, he had the perfect reply. "I have seen four wars in my lifetime. I'm a father of sons; I have a grandson," he said seriously. "I don't ever want to see another generation of young Americans bleed their lives into sandy beachheads in the Pacific, or rice paddies and jungles in Asia, or the muddy, bloody battlefields of Europe."

There are two other lessons for Britain's debaters in Reagan's performance that evening. One is the familiar point that voters prefer the man they might have a beer with to the man who knows all the answers. At first, commentators thought the president had shaded the debate, because "by every measure except aw-shucks niceness", as the pundit Morton Kondracke put it, "Carter was the clearly superior performer". But "aw-shucks niceness" is probably the biggest weapon in a debater's armoury. Brown's handlers will surely have told him that knocking Cameron about the stage, even if only rhetorically, is not enough: a successful debater needs to be liked, not just

respected.

It is no accident, after all, that the best-known line of that 1980 debate was Reagan's half-laughing, half-sorrowful quip: "There you go again." This was an exercise in evasion, a weak joke made after Carter's perfectly good point that Reagan had opposed the Medicare programme back in the 1960s. But nobody remembers Carter's attack now. In a gladiatorial setting where personality counts for more than policies, it is better to be funny and friendly - as George W Bush's aides told him in 2000 - than a grim-faced know-all.

The other lesson from Reagan's performance is that there is no substitute for a good scriptwriter. Reagan's opponents often derided him as a mere actor, but his years in Hollywood had taught him that a performer is only as good as the man who writes his lines. In David Gergen, Reagan had one of the best.

His peroration that night - "Are you better off now than you were four years ago? Is it easier for you to go and buy things in the stores than it was four years ago? Is there more or less unemployment in the country than there was four years ago? Is America as respected throughout the world as it was? Do you feel that our security is as safe, that we're as strong as we were four years ago?" - was devastating in its simplicity, not least because Carter's record made it unanswerable.

There is a lesson in this for Cameron's aides: when times are hard, make the debate a referendum on your opponent's record. There's no need to talk about your own policies if you can score points by mentioning his.

If Brown's aides have any sense, they will have looked closely at Carter's performance that night. It is no good being a master of detail if nobody likes you, and banging on about your experience - Carter mentioned the "Oval Office" ten times in an hour - is generally a waste of time. If voters cared only about experience, the older candidate would always win and Margaret Thatcher would still be in power.

Secret weapon

Above all, though, Carter's performance is a reminder that candidates should stick to the script. In rehearsals, the president had suggested that when the subject of nuclear weapons came up, he could mention his 13-year-old daughter's fears of a world war. His aides thought it was a terrible idea. But under the heat of the lights, and in front of 100 million Americans, Carter decided to do it anyway. “I had a discussion with my daughter, Amy, the other day, before I came here, to ask her what the most important issue was," he said. "She said she thought nuclear weaponry, and the control of nuclear arms."

It was a trite and shameless bid for public sympathy. "Oh my God, not that!" exclaimed Carter's media chief, watching backstage. "It's so bad that it's funny!" There is a place for candidates to mention their families, as long as they do it skilfully. But if the Prime Minister starts telling us what the six-year-old John Brown thinks about the deficit, we will know he is in trouble.

In many ways, the Reagan-Carter clash was a reminder of everything that is wrong with televised debates: their fixation with personality rather than policy, their obsession with the image rather than the word, their emphasis on the individual rather than the party. And yet, those who worry that such debates will come to dominate British elections can take heart from the American experience. The Reagan-Carter debate, held so close to election day, was fresh in voters' minds as they cast their ballots, but by and large these clashes make very little difference to the result. Most of them are constipated affairs. Is there anyone out there who remembers the two Clinton-Dole debates, or can quote a single line from the three Bush-Gore encounters, or would voluntarily watch the three Obama-McCain clashes again?

Top of the pops

Even the debates that have gone down in history were far less interesting than is often recalled. All that voters learned from four hours of Kennedy-Nixon in 1960 was that they had the same policies but Nixon wore less make-up. To be sure, television viewers thought that Kennedy had won, but the narrowing of the polls afterwards punctures the myth that this guaranteed him the election. Even Gerald Ford's insistence in 1976 that there was "no Soviet domination of eastern Europe" did not really matter: after a brief blip in the polls, the president recovered and went into election day neck and neck with his challenger.

If every election produced something like Reagan-Carter, it would be good television but bad politics. Until now, British leaders have resisted debates, and for a very good reason: not only do they score points off one another every week in the House of Commons anyway, but we are meant to live in a parliamentary system, not a presidential one.

Probably the wisest thing anyone has ever said about televised debates came from, of all people, Alec Douglas-Home. Asked if he fancied debating Harold Wilson in 1964, he shook his head. "If you aren't careful, you know, you'll get, what's it called? A sort of Top of the Pops," he said. "You'll then get the best actor as leader of the country and the actor will be prompted by a scriptwriter. I'd rather have our old ways really and put our policies firmly in front of our people."

Let the battle commence.

Dominic Sandbrook's books include "Never Had It So Good: a History of Britain from Suez to the Beatles", published by Abacus (£10.99). He writes the What If . . . column for the NS