John Curtice: May poll is looking increasingly risky for Blair

In calling an election today Mr Blair may be taking more of a risk than he imagines. Our latest NOP poll suggests that Labour's lead has fallen to three points, lower than at any time since last October. While such a lead might be sufficient to deliver Labour a majority of around 80, any further slippage over the next four weeks would probably mean that Mr Blair would no longer win the secure third term he is hoping to enjoy.

In calling an election today Mr Blair may be taking more of a risk than he imagines. Our latest NOP poll suggests that Labour's lead has fallen to three points, lower than at any time since last October. While such a lead might be sufficient to deliver Labour a majority of around 80, any further slippage over the next four weeks would probably mean that Mr Blair would no longer win the secure third term he is hoping to enjoy.

Our poll, the first to be taken since Mr Howard sacked his fellow MP, Howard Flight, suggests that Labour hopes - that the row over his decision had taken the wind out of the Conservatives' sails - are unfounded. Mr Howard's party has emerged from the affair just one point down on its previous reading in the middle of March. As is so often the case, what seemed to matter in the heady confines of the Westminster village has caused barely a ripple among the public.

Not that Mr Howard's decision was popular. By a majority of three to two, the public feel the sacking was wrong. Even among those who say they intend to vote Conservative, only 59 per cent back their leader's decision. The affair has clearly caused dissension within the party's ranks. But so long as it does not re-emerge, it would not appear to have done much damage to the party's standing.

Meanwhile the Conservatives even show signs of having won some of the campaign argument about tax and spend. Voters are still sceptical about the party's claim that taxes can be cut without doing harm to the NHS and schools; only 41 per cent believe they can, while 52 per cent reckon they cannot. But even so, voters are evidently less sceptical now than they were in November when 58 per cent did not believe the Conservatives' claim. But apparent admiration for Mr Letwin's arithmetic is apparently not enough to win votes.

So the ups and downs of the Conservative campaign have made little difference to the party's fortunes. Moreover, in focusing its fire on the Conservative campaign, Labour may have underestimated the threat to its prospects posed by the Liberal Democrats. After an apparent hiccup in the depths of the winter, Liberal Democrat support has now moved back above 20 per cent - and it is that movement that has apparently helped erode Labour's lead.

For every person who voted Conservative in 2001 but has now switched to the Liberal Democrats there are no less than three who have made the equivalent switch from Labour. This suggests that any further increase in Liberal Democrat support over the next four weeks is likely to come more at Labour's than the Conservatives' expense. Rather than persuading voters not to take a step backwards by voting Conservative, Mr Blair's real task may be to persuade voters not to take a step sideways into the arms of Charles Kennedy. Labour also clearly has to enthuse those who say they are inclined to support the party. Although there are signs in our poll that this election might be beginning to attract people's interest - 59 per cent of all voters now say they are certain they will vote, up 5 points on our last poll three weeks ago - those who say they will vote Labour remain relatively disinclined to go to the polls. Only 64 per cent of Labour supporters now say that they are certain to vote compared with no less than 77 per cent of Conservative supporters.

Labour voters are also more hesitant at expressing their support in the first place. Nearly one in four of those who told NOP's interviewers that they would vote Labour only did so after being asked a second time how they would vote. In contrast, this was true of only one in six Conservative supporters. Even then, more of those who say they still don't know how they will vote are people who said they voted Labour last time. Indeed, our estimate of the party's standings makes an allowance for the fact that many of these will probably end up in the Labour fold.

In short, much of Labour's vote is a reluctant vote, evidently disappointed with the record of the past four or eight years, but not sure whether to stick with the party nevertheless, to flirt with the Liberal Democrats or not to bother at all. In calling an election on 5 May, Mr Blair will be staking his future on his ability to persuade this soft Labour vote that it should back him again after all.

John Curtice is professor of politics at Strathclyde University, Glasgow