Following most local elections all the political parties are able to boast a measure of success. One loses fewer seats than expected; another gains more than its own forecasts had claimed to be likely. This year is different.

Labour must make gains or Gordon Brown will have done worse in his first set of contests than Tony Blair did at the height of the Iraq war controversy in 2004, when most of the seats up for grabs on Thursday were last contested.

The Tories must also make gains or David Cameron will face the unpalatable fact that, despite the government’s troubles, support for his party has actually fallen back over the past year. The Liberal Democrats must at least avoid losses or Nick Clegg will begin his term as leader on a downward slope.

On the face of it, the Tories will be shooting at an open goal on Thursday. Their opinion poll ratings are at a 20-year high and they have been the leading party in local government since Cameron became leader.

However, they still need to prove that they can regularly hit the critical 40% mark in terms of popular support at a real election rather than merely in responses to pollsters. That figure is widely accepted as the minimum national vote share likely to be needed for them to win a general election outright. They just reached it in last year’s local contests, but in 2004 fell some way short with 37%.

The Tories’ target must be to make 200 or more gains. That would see them winning scores of seats from both of their opponents and confirm their standing in the polls. Gains of between 100 and 200 seats would be good enough, but would show them dominating the electoral landscape rather less than Labour did in the run-up to its 1997 landslide victory. Any fewer than about 100 gains will suggest that they still have to convince the electorate that they are ready to form the next government.

A crucial factor in determining the extent of Tory progress will be the performance of the Liberal Democrats, who will approach these elections with some trepidation. They lost about 250 seats, mainly to the Tories, in England last year and can ill afford a repeat. Results in line with local by-elections would yield them perhaps 50 or so gains. Most of those would be at Labour’s expense, but such a level of vote would also have a knock-on effect on the Tories. In fact, it is probably necessary for the Lib Dems to lose at least 50 seats overall for the Tories to fulfil their ambitions.

For Labour, the situation is more straightforward. If seats are lost on Thursday, they will experience their worst set of local election results since the nadir of Harold Wilson’s government back in 1968. If those losses can be kept to double figures, the party may just be able to defend the outcome in terms of voter concerns about the economic climate. The closer the losses creep towards 200 – one in every seven seats being defended – the more jittery Labour backbenchers will become about their career prospects.

On the other hand, if the party should make gains – especially if accompanied by a Ken Livingstone victory in London – the leadership will claim it has begun to put more than six months of bad headlines and internal unrest behind it.

With only a third of councillors being elected in most authorities in England, there is little scope for dramatic turnovers in council control. However, the Conservatives do have some councils in their sights.

A 4% swing to the Tories will see Labour lose overall control of Reading for the first time since 1986. A similar movement of votes in Nuneaton and Bedworth would turn the Warwickshire council from Labour to Tory. In Wales the Tories will hope to win Vale of Glamorgan to add to Monmouthshire, which they took in 2004. In each case spice is added to the contest by the fact that the councils encompass critical Labour-held parliamentary marginals.

The authors are directors of the Elections Centre, University of Plymouth

Quick guide to Thursday’s polls

WHAT’S HAPPENING? Elections will be held on Thursday in 155 councils and four new unitary authorities across England and Wales. Most will have a third of their seats up for grabs. In London, there will be voting for the mayor and the Greater London Authority

HOW DO I JUDGE THE RESULTS?

Tories: More than 200 gains and Downing Street begins to beckon for David Cameron. Between 100 and 200 gains is a positive result but hints that there is more work to be done. Anything less than 100 would be very disappointing

Labour: More than 100 gains would be amazing, but any positive result would suggest the party may have bottomed out. Losing seats would leave an electoral mountain to climb

Times Recommends

• Shrinking violet Gordon turns into raging bull
• Eddie Izzard: 'I keep thinking if I do all these things she'll come back'
• Is sex for the disabled the last taboo?