Blair's hopes of surviving until 2008 may rest on local elections

The local elections on 4 May could have a big impact on national politics and will provide a test for the leaders of all three main parties.

For Tony Blair, the battle for control of England's town halls will be the most important local contest since he became Labour leader in 1994. The results could decide whether his party allows him to remain Prime Minister until 2008 as he apparently wishes.

Heavy losses could easily trigger demands for Mr Blair to go sooner rather than later from Labour MPs jittery about their own prospects at the general election. If the calls extend beyond the "usual suspects" who criticise him, he could be in trouble.

A total of 4,360 seats are up for grabs in 176 of England's 386 local authorities, mainly in towns and cities. In most areas only a third of the councillors come up for re-election, making London the main battleground because all 1,861 seats on the 32 boroughs are being contested. There are no elections in Scotland or Wales this year.

London could provide the biggest headache for Mr Blair. When the boroughs were last fought in 2002, Labour was in a much stronger position in the opinion polls than it is now, so the party could suffer significant losses.

Labour, which usually trumpets national issues such as health and education in local elections, will go into reverse when Mr Blair and Gordon Brown launch its campaign today. It will highlight local issues such as crime, antisocial behaviour and the environment. Independent experts believe more than 200 overall losses for Labour would be a bad performance - and enough for Mr Blair's critics to demand his early exit.

The elections will also prove a first major test for David Cameron since he became Tory leader last December. Election experts admit it will not be easy for him to make sweeping gains because the Tories are already the largest party in local government.

"Far from shooting at an open goal, David Cameron's first nationwide electoral test in May sees him having to defend something of a Conservative high water mark," said Colin Rallings and Michael Thrasher, directors of the Local Government Chronicle Elections Centre at Plymouth University. In London, the Tories did relatively well in last year's general election and so Mr Cameron cannot have any excuses if he fails to make real progress.

Overall, election analysts believe the Tories should be looking for an extra 300 councillors in England to be able to claim victory.

There is already three-party politics in the town halls, where the Liberal Democrats have 4,300 councillors and control 34 authorities. In his first national test, Sir Menzies Campbell hopes to improve on both - but his officials are not disclosing their numbers. The Liberal Democrats will be looking to capitalise on disaffection with Labour in its heartlands. They will hope to show that gains from Labour at last year's general election were not a one-off protest vote over the Iraq war. According to the analysts, 100 overall gains would make it a good night for Sir Menzies while net losses would be seen as a bad performance.