Marginal seats make case for caution

Most recent polls have typically put Labour between six and eight points ahead. Such leads seem to suggest Mr Brown can go to the country confident of a ringing personal endorsement from the electorate.

But if Mr Brown wants a "personal mandate", he needs to do more than win. He has to do as well as Mr Blair did in 2005 – an overall majority of 66 – and preferably better. Anything less will be regarded as a setback, not a mandate.

Of course, a six- to eight-point lead is well above the three point lead Mr Blair secured two years ago. But the next election will be fought in England and Wales on new boundaries. These do Labour no favours.

Estimates from Plymouth University suggest that if used at the last election they would have cut Labour's majority to 48.

So Mr Brown has to win more votes than Mr Blair did last time just to stand still. To secure a majority of 66 he may need a lead of just over four points, not three. That may still be less than six to eight – but the difference leaves little room for error. What if Labour lost a little ground in the campaign? What if the polls are overestimating its lead a little – as they have done at most recent elections? And, above all, what if Labour's national lead is not reflected in the marginals?

Our chart shows the key marginals – the 20 most marginal Labour seats the party cannot afford to lose, and the 20 seats Labour is best placed to capture, half of which it must gain to reach 66. Each could change hands on a small swing either way – underlining how thin the margin between success and failure could be for MrBrown.

Meanwhile on Thursday night Mr Brown received less encouraging news. In three county council seats, all last fought over on the same day as the 2005 general election and all in key marginals, there were sharp swings to the Conservatives – on average by four points.

This month's local by-elections point at most to a narrow Labour lead, not a large one –not what Mr Brown needs at all.

John Curtice is Professor of Politics, Strathclyde University