On 15 April 2010, the first televised leaders’ debate in a British general election campaign took place – an event that is best-remembered today for the accidental soundbite: “I agree with Nick”.

The UK was unusual among major democracies in not holding televised leaders’ debates until 2010. The United States held its first televised presidential debate fifty years previously in 1960 (John F. Kennedy vs Richard Nixon).

As such, the first of three debates, between the Labour prime minister, Gordon Brown, Conservative leader David Cameron and Liberal Democrat leader Nick Clegg, was a major media event.

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Clegg was widely seen to have outdebated his opponents.

His revelatory performance even inspired a political phenomenon dubbed “Cleggmania” which, temporarily at least, upended the established two-party dynamic.

As the least known of the three leaders, Clegg seized the opportunity provided by the debate to present himself as a fresh alternative to the “old parties”. For much of the event, Brown and Cameron focused their attacks on each other, creating space for Clegg to exploit.

At one point, the Lib Dem leader interjected: “The more they attack each other, the more they sound exactly the same.”

He declared: “Don’t let them tell you that the only choice is between two old parties that have been playing pass the parcel with your government for 65 years now making the same old promises, breaking the same old promises.”

Clegg referred to “these two”, denigrating Brown and Cameron simultaneously. In his closing statement, he urged voters to say “yes to something new”.

Brown and Cameron’s strategy for handling the Liberal Democrat leader – to “agree” with him – effectively elevated Clegg’s status to that of an equal.

The immediate impact of the debate was a dramatic surge in support for the Liberal Democrats. An instant YouGov poll published after the debate found viewers judged Clegg the clear winner, with 51% naming him as the best performer (29% for Cameron, 19% for Brown).

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In the days that followed the debate, Cleggmania fundamentally altered the dynamics of the campaign. The Conservative and Labour parties were forced to directly respond to the third party’s new prominence. All of a sudden, Liberal Democrat policies received a level of media scrutiny usually reserved for the two main parties.

Responding to Clegg’s performance in the 2010 debate, Cameron commented: “I think ‘a plague on both your houses’ is a very good song.”

A YouGov poll, published on 20 April, placed the Liberal Democrats on 34%, one point above the Conservatives and six points ahead of Labour.

Another opinion poll suggested Clegg was almost as popular as Sir Winston Churchill.

In the 2010 election, Clegg campaigned on a platform of restoring trust in politics. One memorable Liberal Democrat election broadcast urged voters to “Say goodbye to broken promises”.

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Speaking directly into camera, Clegg stated: “Broken promises – there have been too many in the last few years… Our nation has been littered with them, a trail of broken promises… This election is different to other elections. The trail of broken promises can come to an end.”

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The second debate, held on 22 April, was a feistier, more evenly matched contest. Brown and Cameron were better prepared for Clegg’s line of attack.

Cleggmania, in the end, was an ephemeral phenomenon. The party won 57 seats, down 5, at the 2010 general election; its final share of the vote (23%) marked a one point improvement on the 2005 election.

However, in the hung parliament that resulted from the 2010 campaign, Clegg’s party held the balance of power. The Liberal Democrats entered coalition with Cameron’s Conservatives and Clegg secured the post of deputy prime minister.

A study by Oxford’s Reuters Institute for the Study of Journalism into the 2010 televised debates found that around two-thirds of respondents said they had learned something new from watching, while roughly three-quarters felt they had gained a better understanding of the personal qualities of the party leaders. Large majorities – ranging from 58% to 70% – also reported that the debates helped them understand party policies more clearly.

The impact was particularly pronounced among younger voters. 55% of those aged 18 to 24 said the debates made them “more interested” in the campaign, a higher proportion than in any other age group, and they were also the most likely to discuss the debates with others.

In his 2019 memoir, For The Record, Cameron said that before the debate he had “never been so nervous in my life.”

He wrote: “The news channels covered the build-up as if it was England in the World Cup final. Brown, Clegg and I stood on a primary-coloured set like gameshow contestants. As predicted, Clegg was painting the blue and red parties as the old guard, and himself as the new kid on the block. It seemed a breeze for him.”

Cameron added: “I was bad. Not switch-off-the-telly, hide-behind-the-sofa bad. But aloof and stiff. Lacking passion. Anecdotes that were too contrived. And one bit of absolutely essential preparation that I failed to put into practice was properly looking down the camera when I spoke.

“Colleagues and friends were polite afterwards, because while Clegg had undoubtedly won, at least I hadn’t lost (that honour went to Brown).”