John Curtice: Battle narrows down to the key marginals

The last few weeks have witnessed a transformation of the election battle. Between the conclusion of the party conference season and the end of January, the Conservatives had averaged 12 points ahead in the polls, sufficient to give them an overall majority. But for the last fortnight or so, a flurry of polls has put the Tory lead over Labour at no more than half that figure. According to the conventional calculations, a six-point lead will only just be sufficient for the Conservatives to be the largest party in a hung parliament.

However, those conventional calculations are based on a crucial assumption: that the ups and downs in party support are the same everywhere. But if the Conservatives could secure a higher swing in those seats where Labour's lead last time was smallest they might be able to secure an overall majority on much less than the 11-point lead they would otherwise require. Little wonder, then, that attention is increasingly focusing on what is happening in these key marginals. After all, the Conservatives are concentrating their organisational effort in these seats.

Meanwhile, Liberal Democrat supporters in such seats may be less willing this time to vote tactically for Labour to keep the Tory out.

Three polls published in the last few days have all suggested the swing to the Conservatives is indeed higher in Labour/Tory marginals. That difference is potentially crucial.

The Conservatives need a 6.9 per cent swing from Labour if the Liberal Democrats maintain the vote they won in 2005. But national polls put the Liberal Democrats on 18 per cent, down five points on last time.

If the Conservatives can profit from any Liberal Democrat decline, they would need to make fewer gains from Labour. The battle for power does not just depend on what happens in Labour/Conservative marginals. Tory prospects could also depend on whether they can also knock down some Liberal Democrat pedestals. And as to whether he is on course to do that we, as yet, know relatively little.

John Curtice is professor of politics at Strathclyde University.