John Curtice: Make no mistake, this is still a three-horse race

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David Cameron and Gordon Brown must now be asking themselves a simple, but crucial, "if only" question. If only last week they had prepared properly to debate with Nick Clegg as well as between themselves, might they have avoided the Liberal Democrat surge that has apparently transformed this election.

After the débâcle of the first debate, both were ready to attack Mr Clegg in Thursday's second joust. This helped ensure both men did enough to persuade most of their existing supporters at least they had performed best.

According to ComRes's instant poll of viewers of the second debate, 75 per cent of those who declared beforehand that they were Labour supporters reckoned Mr Brown performed best. Similarly, 67 per cent of Conservative supporters put Mr Cameron ahead. Last week only half of Labour and Conservative supporters put their man first.

So neither man gained much direct advantage over the other on Thursday, but at least both held their own. And if they had managed that last week, then the Liberal Democrat surge would probably not have happened. The election would now still be looking primarily to be a tussle between the two of them.

However, putting the genie back in the bottle is more difficult than stopping it escaping in the first place. Nick Clegg may not have gained much advantage either on Thursday but he, too, did largely manage to hold his ground. Among Liberal Democrat supporters, 64 per cent thought Mr Clegg was the winner – and of course that group is around 50 per cent bigger now than it was just a week ago.

It thus comes as little surprise that the average rating of the leaders across all five of the post-debate polls conducted among viewers on Thursday night was almost exactly the same as their parties' average rating in the pre-debate polls.

So, with Labour currently running at 27 per cent, Mr Brown was reckoned to be the winner by 27 per cent. Similarly, with his party on 33 per cent, Mr Cameron also secured 33 per cent support. Only Mr Clegg's figure was slightly different – he, like Mr Cameron, managed 33 per cent, but this was slightly ahead of his party's current rating of 31 per cent.

The second debate was, therefore, a no-score draw. Mr Cameron matched Mr Clegg while Mr Brown was not far behind. Even Mr Brown had some consolation. At eight points, the increase in his average score almost matched the nine-point improvement in Mr Cameron's rating.

Consequently, it is unlikely this debate moved many votes in any one particular direction. Indeed, ComRes's estimate of the post-debate voting intentions of those who watched on Thursday is almost identical to its equivalent estimate among viewers the previous week.

If so, then the election remains a three-horse race and the odds on a hung parliament have shortened significantly. On the current poll ratings, both Labour and the Conservatives could expect to win around 250 seats, the Liberal Democrats some 110. In that event, Mr Clegg would certainly not be at risk of being ignored.

John Curtice is professor of politics at Strathclyde University, and is analysing the election opinion polls for The Independent