Andrew Vine: Haunted Brown is laid low by the curse of his Bigotgate blunder

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This was his hole card – the final debate, his pet subject. The theme was the economy and Gordon Brown has spent the past 13 years trumpeting his genius, the last three of them his foresight and decisiveness in rescuing the banks. This should have been the moment to appear the man to trust, a giant in comparison to the callow beginners at the other lecterns.

But that was before Bigotgate and the gaffe, which if he loses the election, will follow him to his political grave. All politicians have a moment, for good or ill, that defines them. For his predecessor, it was the calamitous decision to invade Iraq; for Mr Brown, it had come in the rather less exotic setting of a street in Rochdale, and the realisation of it was there in everything he said and did.

He hit the subject head on, declaring in his opening statement that he did not always get everything right. He was correct in that; he got precious little right in this climactic encounter. The knowledge of the magnitude of the gaffe hung over him like a personal cloud; he looked harassed, rattled and tired. That strange, forced smile was more than usually strained, the hand gestures just a little too emphatic and marionette-like.

There was a hint of panic about him that had been absent in the previous two debates when he had, predictably enough, gone on the attack but scored points off his appointments.

Mr Brown had plainly been closely coached, and it rendered him robotic and repetitive. Prefabricated slogans tripped off his tongue – double-dip, return to the 30s, 80s and 90s, global financial levy, race to the bottom. Oddly, for a debate on the economy, he kept his liking for reeling off strings of statistic on a tight leash on the night that it might have helped him steamroller his rivals.

The whole thing was more aggressive than it had been in either of the previous two weeks. It had to be. There was no margin for error left, no second chance. This was be the final, lasting impression that all three men would leave on the mass television audience.

It was now or never for Mr Cameron to make the impression that had not quite managed in the first two debates, and at last, he did. He never landed a knockout blow on either of his rivals but finally achieved what he had striven for – to look statesmanlike and a credible Premier-in-waiting.

Mr Brown's gaffe had helped – even if Mr Cameron was was still slightly tense, he appeared controlled and thoughtful in comparison. The tautness had gone, he was animated, engaging and forensic in his criticisms of the Prime Minister.

Mr Brown did the Conservative leader a huge favour by directing most of his fire at him; it gave Mr Cameron the floor and the focus, and in comparison to his haggard opponent looked fresh and full of ideas.

Neither of them, though, could look as fresh as Nick Clegg. If he chooses to leave politics at some point, a lucrative career awaits in coaching those faced with make-or-break television appearances. Once again, he had been fortunate enough to draw the lot for the central lectern, and span an effective new twist on his position, gesturing simultaneously to left and right at his opponents, bracketing them together as predictable and bereft of ideas.

He was vulnerable to allegations that the Lib Dems would be soft on illegal asylum seekers, but danced through the difficulties, just as he had on Britain's nuclear deterrent last week. Better, he spoke to viewers who have to deal with compromise and consensus every day of their lives by suggesting that the political parties did likewise by putting their heads together and sorting out the country's finances.

It was an unanswerable point. He appeared reasonable, sensible and with just enough anti-establishment spirit to set him aside from what he termed the "two old parties".

Ultimately, it was a heat. In the first debate, Mr Clegg had taken advantage of his relative unfamiliarity to race away with an undisputed victory; in the second, he had nicked it as Mr Brown thundered up close behind him. He should be satisfied with the draw he played last night with the Conservatives; at last, at long last, Mr Cameron found his form and turned in a performance that communicated with viewers at home.

How different in might had been if Mr Brown had not forgotten that he had been wearing a microphone a day before; how much better if he had simply not said what he had, whether he thought it or not. Bigotgate hung at his shoulder all last night; as he tried to talk to viewers about looking forward, his mind seemed to be haunted by what had happened. And that gave him the air of yesterday's man.