The most famous TV election debate image is a sweaty and unshaven Richard Nixon up against the telegenic JFK in the US in 1960.

Here in the UK, the most decisive debate game-changer was Gordon Brown and David Cameron conceding “I agree with Nick” in 2010.

That sparked the “Cleggmania” which propelled Nick Clegg to the post of deputy prime minister in Mr Cameron’s coalition government.

Mr Nixon’s disaster under the harsh TV lights came in the first TV debate held in a US presidential election, one of four during the 1960 campaign.

At the time, Mr Nixon was Republican vice-president and John F Kennedy a young Democratic Party senator. But Mr Nixon was cruelly exposed as a TV novice and looked shifty.

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He was pale, after a hospital stay because of a knee injury, his suit colour blended in with the set and his refusal to wear TV make-up revealed a five-o-clock shadow.

The debate was a turning point in the campaign and changed US politics for ever: just as the 2010 Brown-Cameron-Clegg debates blazed a trail in the UK.

It was 50 years after the Nixon-JFK clash before TV debates between party leaders in general election campaigns began in the UK. There were three in 2010, hosted by ITV, Sky News and the BBC.

Prior to 2010, the lack of debates wasn’t for the lack of trying, however, with opposition leaders or prime ministers behind in the polls issuing a challenge and then being rebuffed.

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As a general rule, it’s usually underdogs with nothing to lose – like Rishi Sunak in this campaign – who want to debate against an opponent with a healthy lead in the opinion polls.

But not always. In 1964 Labour’s Harold Wilson challenged Alec Douglas-Home. No, said the Tory prime minister, it would be like “Top of the Pops”. But in 1970 prime minister Wilson said no to Edward Heath.

In 1979, a struggling James Callaghan, who looked to be on his way out, challenged Tory leader Margaret Thatcher. But this time it was the opposition leader who turned down the prime minister.

As prime minister, Mrs Thatcher also said no to Neil Kinnock in 1987, as did Sir John Major in 1992, even though he was behind in the polls – before pulling off a shock election victory over Labour.

Later, Sir John observed: “Every party politician that expects to lose tries that trick of debates and every politician who expects to win says no.” That’s still true today.

The first debate in 2010 was at the old Granada studios in Manchester, home to the Coronation Street set and, back then, a replica House of Commons used for TV dramas.

As well as the “I agree with Nick” errors by Mr Brown and Mr Cameron, a Sky News body language expert said Mr Clegg looked strongest because he looked directly at the TV camera lens.

The Sky News debate, chaired by Adam Boulton, was held in an arts centre in Bristol Harbour. This time there was no “I agree with Nick”. Mr Brown and Mr Cameron had learned their lesson.

The BBC debate, in the Great Hall of Birmingham University, was held the day after Gordon Brown’s most disastrous day during he 2010 campaign, his “bigoted woman” gaffe.

Campaigning in Rochdale, the then-prime minister had been heckled and challenged on immigration by a voter, Gillian Duffy, but left his lapel microphone switched on when he was driven away.

“That was a disaster – they should never have put me with that woman,” he said, berating his staff. “Whose idea was that? Ridiculous.” Asked what she had said, he replied: “Everything, she was just a bigoted woman.”

The next day, at the start of the TV debate, he said sheepishly: “There’s a lot to this job and as you saw yesterday I don’t get all of it right.”

But once in Number 10 it was Mr Cameron’s turn to have regrets. And asked by Sky News in December 2012 if he would commit to TV debates at the next election, he signalled he wanted changes before agreeing.

“My reflection on last time was that they did suck all the life out of the campaign,” he said. “The press and all of us were interested in the run-up to the debate, the debate and the post-debate analysis, not the rest of the campaign, which I really enjoy.”

So by 2015 the format had changed. In the first programme, Mr Cameron and Labour’s Ed Miliband separately answered questions from Jeremy Paxman and then from an audience, moderated by Sky’s Kay Burley.

Second was an ITV debate with seven party leaders, including Mr Cameron, Mr Miliband, Mr Clegg, Nicola Sturgeon and Nigel Farage, and third a BBC debate with five opposition leaders but no Mr Cameron and no Mr Clegg.

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Fourth was a BBC Question Time with Mr Cameron, Mr Miliband and Mr Clegg interviewed separately. And Mr Cameron’s tough negotiating tactics paid off as he won an overall majority and the Lib Dems suffered major losses.

In 2017 Theresa May refused to take part, later admitting that was a mistake. A well as TV interviews, two debates took place, with seven party leaders invited.

Labour’s Jeremy Corbyn missed the first, but at short notice attended the second, at the Cambridge Union, where Amber Rudd deputised for Ms May, despite her father dying three days earlier.

In 2019, Mr Corbyn and Boris Johnson debated twice, in clashes dominated by Brexit. In the first, Mr Corbyn was laughed at by the audience when he failed to say which side of the argument he was on.

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In the same debate, Mr Johnson – who less than three years later would be forced to quit as prime minister after lying to parliament over “party-gate” – was laughed at for claiming he believed truth matters.

In the second, Mr Johnson attacked Mr Corbyn over his stance on antisemitism, the issue which four years later led to the former Labour leader being thrown out of the party and fighting this election as an independent.

Now it’s the turn of Rishi Sunak and Sir Keir Starmer. Advice to both? Wear plenty of TV make-up, unlike Richard Nixon. And unlike Mr Brown and Mr Cameron, don’t agree with your opponent.

What could possibly go wrong?