Analysis By Brian Wheeler BBC News political reporter

May's local elections in England could be a make-or-break night for each of the three main party leaders at Westminster.

May's local elections will be fiercely contested

For David Cameron, the poll is the first nationwide electoral test since he took over as Conservative leader in December.

The Tories are the largest party in local government, with more than 8,000 councillors, 1,500 more than Labour - but their powerbase is mainly outside the large urban areas.

Mr Cameron knows he has to win back support in the inner cities - in places such as Liverpool, Manchester and Newcastle where the party currently have no councillors - to show that he is broadening the party's appeal and is on track to become the next prime minister.

"For too long Conservatives have been seen as the party of the leafy suburbs," he said at the launch last month of the party's inner city policy task force.

"That's changing. We already run local authorities in urban areas like Trafford, Coventry, Bradford and London, and I now want to step up the pace with a new policy focus on our cities."

Key targets are thought to include Bolton, Birmingham and Chester, where no party has overall control.

The Tories have made big gains in local elections in recent years. If the party fail to make further progress on 4 May, particularly in the Metropolitan boroughs, where a third of council seats are up for grabs, Mr Cameron may find his extended honeymoon period coming swiftly to an end.

A lacklustre performance will give ammunition to those on the right of the party who claim Mr Cameron has betrayed its core support in his rush to the centre ground.

A strong showing, on the other hand, will be seized on by Mr Cameron and his supporters as a sign he not only has the right message, but that it is getting through to ordinary voters.

It will also be seen as a signal that there is more to the Cameron revolution than favourable media coverage and a brief bounce in the opinion polls.

Dynamic leader

The Liberal Democrats' new leader Sir Menzies Campbell also faces his first national test on 4 May.

The party's confidence took a severe knock following the departure of Charles Kennedy and a string of sex scandals.

But its shock victory in the Dunfermline and West Fife by-election in February steadied activists' nerves and allayed fears it would be wiped out at the ballot box.

It still faces a squeeze from a revitalised Tory party, which is openly wooing Lib Dem supporters and candidates, but Sir Menzies will be hoping its familiar brand of "pavement politics" - and his pledge to scrap council tax in favour of a local income tax - will do the business at the ballot box.

The party runs only a handful of the local authorities up for grabs on 4 May but it will hoping to grab power on several authorities where there is currently no overall control, particularly in the shires and Metropolitan boroughs.

Likely targets include St Albans, Bristol and London boroughs Lambeth and Richmond.

Failure to make progress - or a significant reduction in the party's number of council seats - will be seen as a major blow to Sir Menzies' authority and his claim to be a more dynamic and effective leader than Mr Kennedy.

Blair's departure

Tony Blair may be facing the stiffest test of all the party leaders on 4 May. The Labour Party is braced for a bad night.

Voters tend to use local elections to register a protest against the government.

But many in the Labour Party fear it could be in line for an even bigger drubbing than might normally be expected at this stage of the electoral cycle.

Labour was badly shaken by the loss, in Gordon Brown's backyard, of Dunfermline and West Fife, one of its safest seats, which no one in the party appears to have seen coming.

And weeks of negative headlines about secret loans and the Tessa Jowell affair - coupled with continuing uncertainty about Tony Blair's likely departure date - have added to the sense of gloom, with all of these subjects reportedly coming up on the doorstep.

Rival parties will also be making much capital out of Gordon Brown's decision to drop the �200 council tax rebate for pensioners.

Party activists fear losing control of a string of local authorities in Labour's inner London heartland, where it controls 15 of the 32 boroughs.

Tower Hamlets, where George Galloway's Respect Party is trying to take control of the town hall, is likely to be a particular flash point.

Three Labour mayors in London, with large majorities, also face re-election on 4 May, in solid Labour areas.

The party is predicted to fare better - and might even gain a few seats - in the Metropolitan boroughs, where it currently controls 16 of the 36 councils.

But a bad result overall will almost certainly hasten Tony Blair's departure from Number 10, with some at Westminster seeing it as a potential "tipping point" in the story of his premiership.

Mr Blair's main asset, for many in the Labour party, is his proven ability to win elections.

Without that, his authority could be fatally weakened - although ministers will, no doubt, attempt to write off poor performance as a "mid term" protest vote, as they did at Dunfermline.

And all three party leaders will, no doubt, be ready to blame individual bad results on "local factors".