Gordon Brown, David Cameron and Nick Clegg step before the TV camera's tonight for the first of three historic Election debates.

These may be the first debates of their kind on British television but in the USA they have formed an integral part of Presidential election campaigns for decades.

The first encounter - between candidates John F Kennedy and Richard Nixon in 1960 - was seen as a pivotal moment in political and televisual history. It wasn't repeated for 16 years, but from 1976 onwards every race to the White House involved an on-screen match-up between those vying to become president.

Here are a few of the most memorable moments during the debates.

1960: Nixon v Kennedy: The Great Debate

On September 26, 1960, 70 million people tuned in to the first televised presidential debate. The first of four on-screen encounters, the session was given over to domestic issues.

The debate saw a pale, underweight Nixon - he was recovering from a serious knee injury that saw him hospitalised a month before - go against Kennedy.

The Democrat was tanned from his recent spell campaigning in California. Nixon later recounted: "I had never seen him looking so fit."

In contrast Nixon looked unshaven, due largely to the lighting in the studio, and suffered from a famously sweaty upper lip.

In terms of substance the two were evenly matched. Radio listeners even put Nixon ahead in a poll following the encounter.

But he fared less well with the television audience who believed that he lagged far behind the relaxed, charming and good-looking Kennedy. The Democrat went on to take the election by a margin of just 49.7% to 49.5%. A survey found 6% of the electorate said they were swayed by the televised debate.

1980: Carter's no-show

In 1980, organisers of the presidential debate the League of Women Voters (LWV) announced that independent candidate John Anderson had sufficient public support to appear alongside President Jimmy Carter and his challenger Ronald Reagan.

Carter, afraid that Anderson's standing could damage his support, insisted on debating Reagan one-on-one before any three-way televised event.

The LWV refused to back down warning Carter that he would be replaced by an empty chair. In the event, they refused to carry out the threat despite the president's no show, with the two chairs being filled by Anderson and Reagan.

In polls following the Carter-absent showdown, Anderson came off the better.

But sliding support thereafter saw him dropped from subsequent TV debates that Carter eventually agreed on.

1988: Dukakis unrattled over rape question

Michael Dukakis was once seen as a favourite to win the 1988 presidential race against George HW Bush. But his lack of passion when questioned over the hypothetical rape of a loved one saw a reversal of fortunes. Asked by moderator Bernard Shaw: "Governor, if Kitty Dukakis (his wife) were raped and murdered, would you favour an irrevocable death penalty for the killer?".

Calmly the candidate responded: "No, I don't, and I think you know that I've opposed the death penalty during all my life."

It may have been true to his beliefs, but his demeanour in answering did not win over viewers, many of whom saw his lack of passion when confronted with the question as odd. A poll showed that his popularity dropped from 49 per cent to 42 per cent following the debate.

Others, however, criticised Shaw for posing the death penalty question in such a personal manner.

1992: Bush Snr caught clock-watching

1992's presidential debate was the first to see three candidates take part, with billionaire Ross Perot appearing alongside President George HW Bush and governor Bill Clinton.

Bush was already put at a disadvantage with his hesitancy over committing to the debate being seen as evidence that he was running scared.

During the session he seemingly came off worse.

An ill-timed glance at his wrist-watch during a question by a member of the audience over the recession was seen as a display of at best impatience and at worst arrogance.

Clinton tried a different approach, approaching the woman who wanted to know how the recession had personally affected the candidates, he looked her in the eye and said: "Tell me how it's affected you again."