John Curtice: The competence and empathy reminded me of Tony Blair

There is no doubt at all about who won Thursday night's debate so far as the viewers were concerned: Nick Clegg. No less than five polls of a representative sample of viewers immediately after the studio lights went down all found far more people nominating Mr Clegg as the winner than either of his two rivals.

According to Populus, as many as 61 per cent reckoned Mr Clegg the winner. Both ICM and YouGov found 51 per cent thought he was best. Angus Reid gave him a more modest 46 per cent, while ComRes, who conducted the largest exercise of all, contacting over 4,000 viewers, put the figure at 43 per cent.

All five polls found that Mr Cameron came a poor second – on average he scored 24 per cent. Mr Brown was a little further behind with an average rating of 19 per cent. The viewers' judgement must have come as a considerable blow in particular to the Conservative leader, given the widespread expectation in advance of the debate that he would win.

More importantly, Mr Clegg won among those who told the pollsters before the debate they were undecided about what to do on 6 May. For example, the large panel interviewed by ComRes found that 45 per cent of "floating voters" reckoned the Lib Dem leader had won, a picture confirmed by the other polls.

Mr Clegg's performance does seem to have persuaded many of the "undecideds" to back his party. Forty per cent of the undecided said they are now inclined to vote Lib Dem. Only 20 per cent stated they might now vote Labour and just 18 per cent the Conservatives.

Similarly in their smaller exercise among 1,000 viewers, Populus found that 74 per cent of the undecideds said they were now more likely to vote Lib Dem, while only 8 per cent indicated they were less likely to do so.

Mr Clegg's appeal to the undecideds helps to explain why ComRes found that, overall, there had been no less than a 14-point increase in Lib Dem support among viewers following the debate. Even if the debate were not to have any impact on the four in five voters who did not watch it for themselves, that switch is still potentially worth an extra 3 per cent of the total vote.

But at this point a little caution is in order. It is not surprising that immediately after being impressed by the Lib Dem leader, some people say they are now inclined to switch to his party. It needs, however, to be borne in mind that there are still three weeks and two debates to go between now and polling day.

The memory of Mr Clegg's performance on Thursday is bound to to fade – and could even be erased – by what happens in subsequent debates. Thursday has created an opportunity for the Lib Dems to gain momentum rather than an inevitable bonus in May.

How did Mr Clegg create this opportunity? According to Populus by persuading people that he knows how to handle the country's problems while at the same time conveying the sense that he is in touch with his fellow citizens. Apparent competence laced with a degree of empathy – perhaps it reminded some of Tony Blair?

John Curtice is Professor of Politics, University of Strathclyde. He is analysing the opinion polls for The Independent

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