Local elections 2013: All you need to know

The latest big electoral test for the coalition will not deliver a verdict on the performance of the government. Instead the battles to come on May 2nd turn this contest into a bitter warm-up for the 2015 election in the constituencies where the Conservatives and Liberal Democrats are fighting each other. All the bitterness of the Eastleigh by-election will spill out into a contest across the country. Neither side is likely to come out smelling of roses – and it's likely the party which does best at limiting the damage will emerge as the winner of sorts.

These are the shire bits of the shires, the most traditionally small-c conservative parts of the country. It is territory last contested in 2009, when the Tories reached their pre-election high and swept the other two parties away. That means the Conservatives are on the defensive this time around. They are clinging on to nearly 1,500 seats in England. The Lib Dems have nearly 500 and are hoping to make gains.

Labour, meanwhile, will be looking to destroy the Lib Dems in places like Lancaster and Northumberland. Their big test comes in places like Derbyshire and Nottinghamshire, where they failed to prevent Tory candidates winning last November's police and crime commissioner elections. They simply must do better this spring if Ed Miliband is to continue building his fledgling impression as a plausible prime minister.

Miliband's impending northern massacres are limited in scale, however. Most of the counties being fought in 2013 are about as far from Labour's home territory as it gets. In 2009 they only managed to win a tenth of the 2,500-odd seats being contested this time around. So the main battle here is a coalition struggle. It is Eastleigh writ large. It is not going to very pretty.

Liberal Democrat prospects this year are hard to judge. Their strongest hope for a gain is in Somerset, where they have four out of the county's five MPs despite losing control in 2009. Even here they need a five per cent swing away from the Tories, however, so another potential gain – Devon – could well prove beyond their reach.

The party generally performs better in local elections than in opinion polls, in large part thanks to the deep roots the party puts down where it has a strong presence. The Lib Dems will be helped by the lack of university students being contested this time around, too. But questions remain about their ability to fight against the national trend.

Assessing the Conservative performance means delving deeper into the results than the headline net gain figures, however. How they do in southern councils which remain under Tory control regardless are critical. They will hold on to Hampshire, for example, but the Lib Dems have done well here – especially after Eastleigh. The Isle of Wight had been a Lib Dem local authority in the past. Then there's Dorset, a county dominated by Tory parliamentary seats with very small overall majorities.

The results are, above all, unpredictable. This is the first time since 1993 that elections for these counties were not overshadowed by much more significant voting events: the three New Labour general election victories, and then the European elections of 2009. Without their distorting effects we might expect the voting to settle back to something more manageable. But a major redrawing of the electoral map has seen one-third of the seats being contested altered. Boundary changes make it fiendishly difficult to calculate how the parties have actually performed – even gatherings of the most impressive minds in the land on the issue struggle to get their hands around how they fit together. It seems there isn't a right answer, which means all the parties have far more to spin with than usual. When results start coming in on Thursday evening and then (mostly) on Friday, treat their assessments with several pinches of salt.

If the Lib Dem vote does start to struggle, all the Conservatives need to do is make gains is maintain their current vote share. This is where the Ukip question comes into play.

All the indications are they will make substantial gains on their 2009 performance, when they took 14% of the vote share in the wards they contested – roughly a quarter of the total. Recent by-election results put their performance somewhere between ten and 20%. They are thought to be aiming to contest half the seats this time around. Although Ukip won't get many councillors – the party only won seven in 2009, after all – they will be trumpeting their vote share from the rooftops.

Where will Ukip's vote come from? Again, this is unclear. In 2009 the Conservatives won 43% of the vote. Now they are hovering around the 30% mark, leaving a sizeable chunk to be redistributed elsewhere. It's thought Ukip will steal around six per cent from the Tories. But they can be expected to make inroads into the other parties, too. The recent Rotherham by-election, where Ukip's candidate Jane Collins came second, shows the effect they are having in a Labour stronghold. So you could reasonably expect Ukip to make some progress here as well. That leaves the kind of voters who had previously backed the Lib Dems or the BNP as a form of protest. They are the most likely of all to up sticks and fall into Nigel Farage's all-embracing arms.

The Ukip factor is the spanner in the works for Cameron's Conservatives. That, and the resurgent Lib Dems' morale after their Eastleigh hold, make this a struggle with an outcome which is far from clear. It is, admittedly, a sideline to the main Tory-Labour battle which will dominate 2015. But in an era of hung parliaments and the strong likelihood that neither of the main parties will gain an overall majority, every vote counts. 2013's local elections might just offer a window into the way the coalition's internal battle will play out in just two years' time.