Andrew Grice: This is Clegg's chance to shine – or sink

Which political party leader had the most to celebrate on Christmas Day? Nick Clegg. The Liberal Democrats are the big winners from this week's historic announcement of three televised debates between the three main party leaders during next year's general election campaign.

The Liberal Democrats, hovering at just under 20 per cent in the opinion polls, normally put on about five points during an election, when the broadcasters, who often ignore them, are obliged by law to give them equal treatment to the two main parties.

The 90-minute debates will give Mr Clegg a much bigger platform. Charles Kennedy and Paddy Ashdown must be green with envy. It usually takes a Liberal Democrat leader one election just to "get known." Mr Clegg's fatal weakness – that voters simply do not know him – suddenly evaporates. His party's polling shows that when people see him, they like him. Having watched him at one of his 100 town hall meetings, I can see why. Now about 10 million people will see him up close, and not judge him on a few soundbites on news bulletins.

The TV debates add another layer of unpredictability to an election whose outcome is still far from certain. Despite welcoming the announcement, senior Labour and Tory figures are privately nervous about the wild card involvement of the Liberal Democrats. Labour might well have preferred a head-to-head between Gordon Brown and David Cameron. Some Tories think their leader is mad to allow Mr Clegg into the ring, pointing to Canada where the elevation of a third party unhinged the two-party see-saw. But Mr Cameron, who has been pressing Mr Brown for a TV debate for two years, judged that Mr Clegg would have played the victim if he had been excluded, and that it would have been unfair to squeeze him out for political advantage.

Liberal Democrats say they had to form an axis with the broadcasters and fight hard for three, three-way debates, rather than one Brown-Cameron, one Cameron-Clegg and one Brown-Clegg show. No TV company wanted to be drawn out of the hat to stage Cameron-Clegg and all wanted the Brown-Cameron heavyweight championship. So there was agreement at two meetings in the past two weeks for the three bouts involving all three leaders.

The debates are a breath of fresh air in a political system loaded against Britain's third force even though it is likely to win the support of one in four voters at the election. The electoral system is bad enough. Prime Minister's Questions, previously cited as the reason why we didn't need TV debates, offers the Liberal Democrat leader little chance to shine. The Opposition leader goes first, and has six questions to the Liberal Democrats' two. Mr Clegg is deliberately drowned out by barracking Labour MPs. He can never pitch himself against Mr Cameron, so it's no wonder that some voters probably regard him as the Tory leader's little brother. The TV debates will allow him to define himself against Mr Cameron as well as Mr Brown. That will be important in areas like the South-west, where the Con-Lib battle will be crucial.

There is a mixture of excitement and nervousness in all three parties. They will spend many hours "prepping" for the debates. Some of the "ad libs" will have been well-rehearsed. All three will "war game", anticipating how they will be attacked so they know how to respond. Critics say that this – and the media's obsession with a media event – will distort the election campaign. Perhaps. I hope the presenters keep the focus on policy rather than personality. Yet at a time when party politics is regarded with contempt by many voters, there is no valid argument against Britain dragging itself into the modern world, 50 years after America's first presidential debates.

Mr Cameron, a much better communicator than Mr Brown and probably still ahead in the polls when the debates happen, has the most to lose. Yet his aides (like Mr Clegg's) insist that the more people see of him, the more they like him. The recent narrowing of the poll gap may be due to Mr Cameron's slightly lower profile in recent weeks, while the hyperactive Mr Brown is always on our TV screens. The Tory leader will be very visible in January, when he will launch a policy blitz. He believes it will be a very important month.

Mr Brown will enter the TV debates as the underdog, with lower expectations on his shoulders than those of Mr Cameron. He will hope that substance will triumph over spin. In a boxing match, one knock-out blow is enough. I doubt that one will be landed. More likely is that one of the leaders commits a headline-grabbing "gaffe", so they will work overtime to avoid that. Caution may prevent fireworks.

For Mr Clegg, the debates are a threat as well as an opportunity. The Liberal Democrats rightly moan about their lack of media coverage between elections. But in some ways, they are lucky to escape the scrutiny given to the big two parties, since their policies sometimes lack credibility.

Mr Clegg will also have to answer the "wasted vote" argument that deters many people from backing the distant third runner in a two-horse race. But he will now have a unique opportunity to do so, and no excuses if he blows it.