Andrew Vine: Middle man stays ahead on points as knockout blow eludes heavyweights

Fortune favoured Nick Clegg last night; the lots drawn for positions on stage had place him in the middle, which made him the centre of attention and gave him the ideal platform to build on his unexpected success in the first, historic, debate. It was a slicker set in the Sky studio than it had been in ITV's, bunching the three men closer together and emphasising that Mr Clegg stands a little taller than his rivals.

And once again, he seemed to stand a little taller in the exchanges as well. He was the most relaxed and natural of the three, in the way he spoke and how he stood. His winning knack of addressing the audience at home by speaking directly to the camera was, if anything, more effective than during the opening debate. Both Mr Brown and Mr Clegg tried to ape it, but neither had anything like the same assurance, the Prime Minister eventually giving up and going back to addressing the people in the studio.

The second debate always posed problems for Mr Clegg. His relative unfamiliarity to many viewers on the first week had served him well, gifting him the aura of freshness, as did his stance – doubtlessly carefully-plotted in advance – as the outsider who seemed to speak for an electorate disillusioned with politicians in general, and those caught with their fingers in the till during the expenses scandal in particular. He had effectively ambushed Mr Brown and Mr Cameron, when neither audience nor protagonists knew quite what to expect, deftly lumping them together as a Tweedledum and Tweedledee perched at opposite ends of a broken-down political see-saw.

He stuck to the same tack, and it worked again, even when Mr Brown and Mr Cameron ganged up on him over Lib Dem policy over Britain's nuclear deterrent, with the Prime Minister landing a stinging blow by urging him to "Get real", which was the closest either of them had come to making Mr Clegg look like a junior partner in these two debates. His luck held; he was saved by the bell when the host, Adam Boulton, cut the discussion short.

It is baffling why Mr Cameron did not impose himself decisively, as he needed to do. Everything in his recent history, and background, should point towards him doing well in these debates. He is a natural communicator who is at home speaking off the cuff, yet he still appeared stiff and overly formal, his anecdotes about meeting people not carrying much sense of conviction. He never looked entirely confident. If he was trying to compose his features to look thoughtful and visionary, he only succeeded in appearing troubled and rattled; several reaction shots caught him narrowing his eyes and frowning, which only reinforced the impression.

Perhaps it was the lighting, or maybe the make-up to stop his skin shining, but whatever the reason, for a second week, his smooth visage looked oddly waxen, his expression strangely wooden.

There was an air of the rabbit caught in the headlights about the Tory leader, a certain something in the eyes that looked very much like worry, or perhaps bewilderment, that after months of dress rehearsals as Prime Minister-in-waiting opposite a badly-wounded incumbent, the public who will decide if that is his destiny seems less than certain that he is suited to the starring role.

This was not going according to plan; he knew it, and it informed his body language.

It was by no means all bad; as he had last week, Mr Cameron's best moments came when he was fired by genuine indignation, this time at Labour election materials that he said were scaring elderly people. Suddenly a sense of conviction showed, suddenly he looked the part he longs to play, but only isolated flashes of passion over the course of a 90-minute debate will not do.

The senior bruiser at the right-hand side of the stage had a good night, better than in the first debate. Mr Brown will never go down as one of the great political speakers, tending as he does towards the lumpen and staid, and considering his natural inclination to snap into bingo-caller mode and start reeling off statistics. His natural instinct is to attack, and that is what he did; it may not be a likeable trait, but it is an honest one and these debates demand frankness. On Afghanistan, defence and Europe, his aim was to sound calm, wise and statesmanlike, and he made a very creditable stab at all three.

At the end, there was a clear winner. These debates favour Mr Clegg's style, and he came out on top even though the policy areas risked his credibility. Mr Brown is learning fast, and ran him a very close race. And Mr Cameron? With two down and one to go, he could be forgiven for being a very worried man.