By Reeta Chakrabarti
Political correspondent, BBC News
The election campaign will see the UK's first televised debates. But while they will be a novelty here, other countries have been holding them since the US led the way 50 years ago... with plenty of lessons for messrs Brown, Cameron and Clegg
NIXON VS KENNEDY
Nixon was judged to have done well on radio - less so on TV
The most celebrated leaders' debate was in the 1960 US presidential election, between then Republican vice-President Richard Nixon and the Massachusetts Democrat senator John F Kennedy.
Nixon had every reason to feel confident. The year before he had been to Moscow, and sparred with the Soviet leader, Nikita Krushchev, on the merits of capitalism over communism. What did he have to fear from this relatively unknown senator?
But Richard Nixon had been ill, and compared with the suntanned Kennedy he looked pale, drawn and unshaven. During the debate he kept glancing up at a clock, giving him the appearance of shiftiness. Those who followed it on radio were inclined to say Nixon had come out ahead. Those watching on TV said the winner was Kennedy.
FORD FALTERS ON EUROPE
Ford called a debate in the hope he could close the gap with Carter
Americans had to wait 16 years to see the candidates square up to one another again. Neither Kennedy's successor Lyndon Johnson nor Richard Nixon when he became president had felt any need to put themselves through the ordeal.
But Gerald Ford, who replaced Nixon after Watergate, was behind in the polls in 1976, and he felt that engaging in debate could help him close the gap with his rival, Jimmy Carter.
Their second debate that year produced what sounded like a monumental gaffe by Ford - he claimed the countries of eastern Europe were not in any way subject to Soviet influence.
In the middle of the Cold War this was an incredible thing for an American president to say. Ford himself never accepted that he had made a mistake. Stranger still, most viewers did not register the gaffe until newspapers started poring over it in the following days.
FRENCH RIVALS LOSE THEIR CALM
Segolene Royal's debate with Sarkozy was unusually lively
In France, the presidential election of 2007 was the first in which a woman had run. More than 20 million viewers, around half of France's adult population, tuned in.
The debate was a test for Nicolas Sarkozy, who needed to appeal more to female voters, and Segolene Royal, who was seen as inexperienced. It was a tense affair, with the candidates raising their voices and finger-pointing and and with cries of "Calmez-vous!" on both sides.
Royal came out badly. But, despite the intense interest that time, such lively debates are a rarity in France.
Strict laws and a deference towards the candidates aspiring to the office of president tend to turn most of them into rather stilted affairs.
WORM TURNS VOTERS
Rudd's Liberal opponents were accused of skulduggery
Other countries have tried to liven things up by using gimmicks and gizmos - partly to increase voter participation, partly to make the debates more entertaining.
Even before the days of blogs and Twitter, technology allowed voters in Australia and New Zealand to express an instant opinion on every word and gesture, through a wiggly on-screen graphic called the Worm.
It worked by TV channels wiring panels of undecided voters to an electronic dial which measured their responses throughout the debates.
The Worm proved very influential - in New Zealand in 2002 it was so positive about one minority party and its leader that they shot from obscurity into a governing coalition.
Many viewers said they loved the Worm, and party managers were soon poring over footage to analyse which words prompted a positive wiggle.
But the parties were also suspicious, especially where the Worm seemed to favour one candidate over another.
This led to a major row during Australia's general election in 2007. The debate was shown live on three channels, but only one, the commercial network Channel 9, was using the Worm.
Kevin Rudd, the leader of the opposition, was doing well - and shortly after the worm appeared, reacting positively to him, the network lost its live feed of the debate.
This led the channel's so-called "Worm moderator" to accuse the governing Liberal Party of sabotaging his station's coverage. Political skulduggery or not, it did not work - Rudd went on to win.
CARTER RUCK BOOSTS REAGAN
Reagan made capital out of Carter's snub to an opponent
In parliamentary systems it is the smaller parties who have to battle constantly for a place in the debates.
But in the United States the question is whether independent candidates should be invited alongside the main contenders, Republican and Democrat.
In 1980 John Anderson, then a liberal Republican, decided to leave his party to run for president as an independent.
He was up against the Republican nominee, Ronald Reagan, and the incumbent Democrat, President Jimmy Carter.
At first Anderson's poll ratings were a high 25%. But the president refused to debate with him.
Then suddenly he struck lucky. Ronald Reagan responded to Carter's snub by saying he would debate with anyone, including Anderson, any time.
Two of the networks pounced on the chance to bring the two men together. Anderson, who is now 88, told me he thought Reagan had done it to embarrass Carter.
It was not all plain sailing for John Anderson - one of his own aides defected to the Reagan camp, and helped to coach him in preparation.
He said he had found Ronald Reagan over-familiar during the debate - calling him "John" while he himself used the formal "Governor Reagan."
It was a great coup. Some 55 million Americans watched - even though John Anderson eventually only got 7% of the vote.
Ahmadinejad used a debate to level charges of corruption
Issues of style and one-upmanship are the least of the problems in countries where political discussion is not taken for granted. Last year, Afghanistan, Mongolia and Iran screened their first ever televised debates.
Strange as it may seem given the eventual course of events, in Iran debates were introduced to generate more interest in the presidential election.
It was a leap in the dark - and there was an assumption that President Ahmadinejad would wipe the floor with his opponents. But he used one of the debates to level accusations of corruption against former President Rafsanjani and his family, when none of them were present to defend themselves.
His performance shocked many - as did the sight of watching, for the first time, members of the country's ruling elite airing their disagreements in public.
The debates generated a huge amount of interest. Rafsanjani was furious at the allegations against him and, in the days leading up to the election, people were galvanised into thinking that voting would actually be worth it this time.
In the event the Iranian election was widely seen as rigged in Ahmadinejad's favour, leading to a mass outpouring of protest which was then violently suppressed.
UK political party leaders are to become TV pioneers
As the UK looks forward to its first prime ministerial debates, it is worth remembering that this is not the first attempt to get them off the ground.
In 1997 the parties and broadcasters got someway down the road, only for negotiations to collapse, with the parties all blaming each other for pulling out.
Conventional wisdom has it that the party which is ahead has little to gain from a debate in which the leader might slip up, and forfeit their advantage.
At the time the Conservatives believed Labour was never really serious because Tony Blair was so far ahead in the polls. Labour in its turn accused the Tories of going back on agreements and walking away.
This time round there seems to be a rare convergence of interests - Gordon Brown lagging in the polls, David Cameron confident he can outdo him on television and Nick Clegg just delighted to be invited to the party.
What difference do debates make to the way people vote? On this there is little consensus.
Some think they only reinforce voters' already formed impressions of the candidates. Others say that if the contest is a close one, they can be significant in deciding the result.
There are also those who worry that leaders' debates devalue political life by making the contest about personality more than about policy.
What is sure is that there will be countless debates about the debates.
In the US the impact of the Kennedy/Nixon encounters 50 years ago is still being chewed over.
Britain may be new to this particular television genre, but the arguments about its effect on our politics will be with us for years, perhaps generations, to come.