John Curtice: Cameron must show he can depose Brown

At first glance, today's local elections look like a heaven-sent opportunity for David Cameron. Our latest monthly "poll of polls" gives his party a record 10-point lead over Labour. So surely he can look forward to a night of substantial Conservative gains?

But there will be a nagging worry in his mind as the polls close. Will his party do well enough to suggest that it really looks capable of deposing Labour from power? He cannot really be sure of achieving that.

In part, his problem lies in history. Most of the seats up for grabs this year were last contested in 2004. Labour might still have been narrowly ahead of the Conservatives in the polls at the time, but the party's local election performance that year was woeful. It won the equivalent of just 26 per cent of the national vote.

Labour even managed to do somewhat better than this in last year's local elections, even though it was by then on just 30 per cent in our poll of polls, seven points behind the Conservatives. So there is no guarantee that the party will do any worse tomorrow than it did four years ago, even if it has fallen back again to just 30 per cent in the polls.

However, even if Labour does not fall back itself, there should still be modest Conservative gains. After all, at 40 per cent the party's local election performance was two points up on what it achieved in 2004. Simply repeating that performance should be sufficient to ensure that most of the councils on our list of "top Tory prospects" fall into the party's hands. So failure to do so would suggest the party has actually made a step backwards – as would significant losses among the rather large number of councils on our "Tory disappointment" list, where the party is vulnerable to a modest adverse swing.

What Mr Cameron wants to demonstrate is that he has actually moved forward over the past 12 months. The polls still give the impression that although the Government is now deeply unpopular, the electorate are still unsure whether to embrace the Tories as an alternative. The party is struggling to push its support consistently above the 40 per cent mark. It is even far from clear thatit has managed to extend its poll lead in the wake of Labour's furious row about the abolition of the 10p tax rate.

So Mr Cameron's real aim is to win more than the 40 per cent of the equivalent national vote he secured last year, and in so doing extend the gap between himself and Labour. If he can win 42 or 43 per cent, he will be able to claim that he has matched some of the performances that Labour achieved in the run-up to its victory in the 1997 general election – and that his course really is set fair for Downing Street.

Such a performance would imply making substantial inroads into our "real Tory advance?" list. These are councils where the swing the Tories need to win implies the party is doing better than last year. Perhaps the biggest prize of all would be Bury – just the kind of northern town the party needs to be able to do well in if it is to win a general election. Do not be surprised if Mr Cameron hopes to be able to spend Friday there celebrating victory in front of the television cameras.

Of course, Mr Cameron is not the only opposition leader who needs to do well today. So does Nick Clegg, in his first electoral outing as party leader for the Liberal Democrats. But his task looks even more formidable. Although his party's poll of polls rating has stabilised at 18 per cent, it is still no better than it was last year and is three points down on the position in 2004 – one of its best local election performances ever. As a result, Mr Clegg is likely to struggle to avoid losing ground to the Tories.

John Curtice is professor of politics at Strathclyde University