Election 2015: The political battleground

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We may not know who will win the next general election but we do know which parts of the country will determine the fates of the political parties. Here is a guide to the political battlegrounds of the 2015 general election.

The killing grounds in any general election, where governments are made or broken, can be found among that minority of parliamentary constituencies - marginal seats - with a history of being won or lost by parties.

There is no fixed definition of a marginal: but if we choose to define them for the 2015 election as seats with majorities of 10% or less that require a swing of 5% for the incumbent party to lose, then there are currently 194 such marginal seats in Britain, of which 82 are Conservative, 79 Labour, 27 Lib Dem, three SNP, two Plaid and one Green.

At the May 2015 general election, the Conservatives need a uniform swing of 2% in order to win the 20 extra seats they require to govern on their own.

In its turn, Labour needs a uniform swing of 5% to gain the 68 seats it requires to win its own outright majority. Broadly speaking, this means that the Conservatives need to win seats from other parties with majorities up to 4%; and Labour has the tougher challenge of gaining seats with majorities of up to 10%.

The national swing quoted for any general election represents the change in the share of the total votes cast between the first - and second - placed parties nationally, compared with their shares in the previous general election.

However, in most elections, within that national average, there is a wide range of swings across individual constituencies; in 2010 there was a swing from Labour to Conservative of 0.5% in the seat of Birmingham, Edgbaston and one of 14% in Barnsley East.

The challenge Labour faces to win a majority in 2015 looks formidable: the necessary 5% swing they need to secure such a victory has been reached only three times out of the last 17 general elections.

However, Labour starts from historically low base; in 2010 the party received only 30% of the national vote. Put simply, while there is no guarantee it will happen, it is easier for a party to recover from a 30% base than it would be to build on a 37% base at the previous election.

Also, one of the big unknowns about the next election is UKIP's share of the vote. Most polling suggests that the party takes votes disproportionately from the Conservatives, and if this happens in the 2015 general election it is likely to make Labour's task somewhat easier.

How many seats are safe?

The great majority of parliamentary constituencies do not change party allegiance in elections.

Therefore, although there will be 650 individual contests in the 2015 general election, the outcome will be determined in a much smaller number. In 12 of the 17 general elections since 1950, fewer than one-in-ten seats changed hands from one party to another. Even in the massive Labour landslide of 1997, some 70% of seats stayed with the parties defending them.

Overwhelmingly, general elections are dominated by contests in seats where, realistically, only two parties are in the running. However, there are a handful of seats where three (sometimes four) parties are very close to each other.

In these few seats the threat to the incumbent can come from two opponents. In the 2010 general election, in the seat of Hampstead & Kilburn the Labour majority over the Conservatives was 0.1% and over the third-placed Lib Dem candidate Labour's majority was 1.6%.

Marginal seats are not evenly spread

Marginal seats are not evenly spread: within England, 15% of seats in the South East have majorities of 10% or less, compared with 51% in the South West. In Scotland, 19% of seats fall into this category, compared with 45% in Wales.

Northern Ireland does not feature in these marginals because we focus on the battleground fought by the Conservatives, Labour and the Liberal Democrats, at least two of which do not contest Westminster elections in Northern Ireland.

As we approach the next general election, the political parties will make claims about their election prospects. When they do, it may be helpful to have a glance at some of the detailed results in these marginal seats before accepting their assertions.

Who has the easiest task?

How easy, when you look at specific cases, will it be for the Conservatives to make a net gain of 20 seats? Will they lose none to Labour who, in 2010, received barely 30% of the national vote? What sort of seats will Labour have to win to gain a clear majority of its own? Ones with majorities of 10%? How easy does that look?

The Lib Dems claim they will hold many more seats than the national opinion polls would suggest. Well, maybe, but they would say that wouldn't they? And what credit do we give that big assertion when we look at the seats with small majorities that they will need to retain?

I do not know, any more than anyone else, what will be the outcome of the 2015 general election. However, I am confident that the result will be determined in scores rather than hundreds of seats and that those are the ones to watch like a hawk.

Source: Rallings and Thrasher, Election 2010 the Official Results

Produced by Ransome Mpini, Charlotte Thornton, John Walton and Marcelo Zanni