John Curtice: Voters who see no difference between parties may stay at home

Mr Howard and Mr Kennedy have perhaps one straw they can clutch as they read our poll's finding that Labour has its largest lead for nearly two years.

Mr Howard and Mr Kennedy have perhaps one straw they can clutch as they read our poll's finding that Labour has its largest lead for nearly two years. Our interviews were conducted as Mr Blair was touring the country revealing his six pledges and as he and his fellow ministers dominated the airwaves from their party's spring conference. Against that backdrop, Labour ought to be doing well.

But even if this does help to account for the large lead, it suggests that Labour has found some effective tunes for its campaign. In contrast, Mr Howard has to face the fact that his attempts to harry the Government on tax and immigration have so far brought him little reward. Meanwhile, ours is the second poll in the past week to suggest support for the Liberal Democrats - still starved of the guaranteed oxygen of publicity the formal campaign will bring - has fallen to below a fifth of the vote for the first time in nearly three years.

Mr Blair is well aware there could be a considerable difference between what the polls say at the beginning of an election campaign and what transpires in the ballot boxes. He needs his party's pre-campaign to generate a sufficiently large lead by the beginning of April so that he can call a 5 May election with confidence. The 12-point lead in our poll is precisely the kind of "comfort zone" he wants.

Indeed, other evidence in our poll suggests he is wise to want a comfort zone. Concerned that Labour voters will stay at home, he has been trying to mobilise them by arguing there are big differences between the Conservatives and Labour. As yet, few believe him. Just 21 per cent think there is a great difference between the two - just four points higher than the record low at the time of the last election. This lack of a perceived difference between Britain's two main parties helped drive turnout down to just 59 per cent in 2001. It could well do so again. Overall, just 55 per cent say they are certain to vote, down two points on the position four years ago. And the majority who do not see a great difference are less likely to say they will vote than the minority who do.

Moreover, Labour voters continue to be particularly unenthusiastic about going to the polls. Just 59 per cent say they are certain to vote compared with 64 per cent of Liberal Democrats and 69 per cent of Conservatives.

Meanwhile, our poll supports recent speculation that Liberal Democrat voters may be less willing to vote tactically for Labour to try to keep out the Conservatives. At the time of the last election, for every two Liberal Democrat voters who said the Conservatives were their second preference, five backed Labour. Now only three do so. Mr Blair may still have work to do to hold on to all of his party's crucial marginal seats.

John Curtice is professor of politics at Strathclyde University