By the BBC's Gordon Corera

This week in America, TV viewers were offered a choice of political debate.

On CNN they could watch Democrats Bill Bradley and Al Gore spar in Los Angeles, outlining their positions on the role of the Supreme Court in American public life.

The same evening, viewers on NBC and CBS could watch Republicans John McCain an George W. Bush undertake the far more arduous task of being grilled by Jay Leno and David Letterman on their late night comedy-chat shows.

And its not hard to guess which appearances had the bigger audience and which one the candidates would have been more scared of.

After much cajoling Hilary Clinton underwent her baptism of fire on Letterman earlier in the year, in an appearance which most considered a triumph.

But Bush this week suffered from not being in the studio with Letterman, and that, coupled with a delay on the satellite link, meant that the kind of quick interplay and joke-making that normally makes up the dialogue was stunted and interrupted.

Bush also managed to make a gag about Letterman's recent heart bypass surgery which sank like a lead balloon.

Meanwhile, McCain was more relaxed on Jay Leno's show, displaying his luck shoes and an array of trinkets and lucky charms he carries round with him. McCain even did his impression of Leno himself - one he often does at campaign rallies.

But it still begs the question as to why the candidates put themselves through the potential ridicule of these events.

The truth is that these shows will affect public perceptions of political candidates far more than the millions of dollars spent on TV commercials and often will have more impact than coverage on the evening news broadcasts.

The stereotypes of candidates that these shows build up with their nightly monologues create lasting impressions.

Campaign advisers know, and fear, the power of these shows, but they also know that they provide one of the best opportunities for politicians to transform their images - by showing they can lighten up and crack a gag about themselves.

Al Gore talked about how stiff and boring he was while McCain the other night joked about his famous temper ("they awarded me the miss congeniality title in the Senate", he said).

The candidates also know that the audience for this type of show is made up of young people - who are normally turned off by politics.

Bill Clinton paved the way, and it seems unlikely anything will match his masterful 1992 appearance on the Arsenio Hall show, when he donned a pair of sunglasses and played his saxophone to an audience that couldn't quite believe that this guy was running for President.