John Curtice: Northern losses could be fatal for Blair

Events have conspired to make today's local elections primarily a potential pitfall for Tony Blair. But the contests are just as vital for David Cameron and Sir Menzies Campbell. They will be the first test of how well their parties are faring under their leadership. Both will be keen to demonstrate that they are leading their parties to new heights.

Neither opposition leader has a particularly easy task - because their respective parties fared reasonably well in previous local elections. In the last local election round in 2004, the Conservatives performed at a level that equated to securing 38 per cent of the vote in a general election - far higher than the 33 per cent they actually won in the real general election the following year.

The Liberal Democrats have long since been the masters of the art of local election campaigning, regularly winning a higher share of the vote in local elections than in general elections - even when they are held on the same day. In 2004 they scored the equivalent of 29 per cent of the vote, far higher than the 23 per cent the party won in the 2005 general election.

Mr Cameron needs to break the 40 per cent barrier while Sir Menzies has to pass the 30 per cent mark. They are unlikely both to succeed - because, in part, fulfilment of their ambitions is likely to rest on how well they perform against each other.

Labour may currently be unpopular - the party's average Westminster poll rating was no more than 33 per cent, even before last week's Clarke/Hewitt/Prescott crisis. The post-crisis figure seems to have slipped a little further to 32 per cent. But the party was almost equally unpopular at the time of the 2004 elections, when its average poll rating was just 34 per cent. There are therefore unlikely to be further rich pickings to be had from a further fall in Labour support. In any event, both leaders need to demonstrate that if Labour is the problem, they are now uniquely regarded as the solution. So, at the very minimum, the Conservatives will want to register a 2 per cent swing from the Liberal Democrats to themselves - enough to bring them at least 50 gains. Given that even if the Tories do no more than emulate their 2004 performance they should pick up at least 150 seats in those places where the seats up for grabs this time were last contested in 2002 rather than 2004, this means Mr Cameron is looking for at least 200 gains today - while something closer to 300 would be more convincing.

Equally, the Liberal Democrats will want to register at least a 2 per cent swing from the Conservatives to themselves. Such a swing would actually reap a rather larger dividend - around 100 seats - than it would for the Conservatives. As the party could also pick up another 50 seats simply by emulating its 2004 performance in those places where the seats being contested today were last contested in 2002, a smile of delight might legitimately cross Sir Menzies' face if his party makes 150 gains.

Of course, the battle is not just for council seats, but also for the control of town halls. Our map shows where the key battlegrounds are. They are divided into three groups. "Easy targets" are those councils that are likely to change hands if Labour performs almost as badly today as it did in 2004, while there is no net swing between the Conservatives and the Liberal Democrats. "Tough targets" are councils that would change hands if Labour did a little worse than in 2004 - or if, conversely, they managed to emulate their 2002 performance - or if there were as much as a 3 per cent swing from Tory to Liberal Democrat, or vice versa. We also show some potential "big" victories and defeats.

London is the key battleground. All the seats are up for grabs in the 32 London boroughs, whereas elsewhere usually only a third of the seats are. No fewer than five of the six "easy targets" for the Conservatives are in the capital, together with two of three most probable Liberal Democrat gains, both of which depend on swings from Labour.

Nevertheless, if Labour does have a truly bad night it will start losing ground outside the capital, too. The party's position in a number of former strongholds such as Barnsley, Hartlepool and Warrington has been eroded to such an extent they can no longer be regarded as secure. Losing a number of marginal London boroughs would be painful for Mr Blair. Losing former northern strongholds could be fatal.

John Curtice is professor of politics at Strathclyde University

The poll facts

* Elections for 4,360 council seats in 176 councils held today.

* All 32 London boroughs and 144 other local authorities will go to the polls.

* Hackney, Watford, Newham and Lewisham will hold mayoral elections.

* Elsewhere in the country, councillors are elected by three main systems. To check which system your council operates, visit: www.electoralcommission.org.uk/elections/2006schedule.cfm

* The average turnout in elections in May 2002 was 32.8 per cent. In May 2003, 35.6 per cent.

* In the metropolitan councils Labour has 821 candidates, Conservative has 772, Liberal Democrats 708 and others 875.

* In the shire districts the Conservatives have the most candidates with 1,326, Labour has 1,172, Liberal Democrat has1,052 and others 782.

* The Green Party is fielding 1,251 candidates, the BNP 363 candidates, UKIP 319 candidates and Respect - the Unity Coalition 162.

* Polls will be open between 7am and 10pm.