Gordon Brown will study polling evidence from about several dozen marginals before deciding whether to call an election, but one thing is certain – the battle for these seats has never been so fierce.

David Cameron accused the Prime Minister this week of trying to appeal to 4 per cent of voters in the marginals with “dog whistle” messages on immigration and crime, wrapped in the Union Jack. But the Conservatives, too, are focused on winning support in a relatively small number of swing seats where they have most chance of beating Labour.

Their policy of raising the inheritance tax threshold to £1 million was tested exhaustively in focus groups of floating voters from marginal constituencies. It is designed to appeal precisely to voters for whom rising house prices have made inheritance tax a worry – elderly couples who are comfortably off but not rich.

The Tory pledge to cut stamp duty for first-time buyers is similarly targeted at younger voters and their anxious families, in the same affluent areas.

Many of these battleground seats are concentrated in three clusters: the South East, especially in the “M25 corridor” in north Kent; commuter towns in the Midlands linked by the M1 motorway; and a crop of seats with slim Labour majorities in West Yorkshire.

If an election is called, both Mr Brown and Mr Cameron would probably spend much of their time campaigning in these seats. It is no coincidence that Mr Cameron launched his campaign for the local elections in Dartford, Kent, which Labour holds with a majority of only 706.

There are other marginal seats, but as many of these are contests between Tories and Liberal Democrats, they will not impact directly on Mr Brown’s majority provided that Labour remain the largest party in the Commons.

Conservative party polling shows that its support in target seats is running ahead of that picked up in national surveys, even though they are not radically different in character: there is no single issue or group of issues that is of greater concern to marginals than to the electorate nationally. Labour’s polls also show the Tories doing better in marginals, although strategists say that the difference is small.

Lord Ashcroft, the deputy party chairman, is expected to pump millions of pounds directly into the key marginals as director of the Conservatives’ target seats operation.

Senior Tories insist that decisions on which seats get more money are not taken by Lord Ashcroft alone but by the team of politicians and strategists in charge of election planning. But the campaigning techniques encouraged by Lord Ashcroft before the 2005 election, when candidates would bid for grants, remain central to the Tory fight.

By choosing candidates early, and in many cases reselecting people who fought the same seat in 2005, the Conservatives hope to reap the benefits of almost two years of high-profile local activity in their battleground seats.

One advantage of an early election for Labour would be to prevent the Conservatives from outspending them in marginal seats. But sitting MPs tend to have a higher profile than challengers and enjoy a powerful “incumbency factor”, from a regular profile in local media and the ability to spend public money. From April, MPs have been able to spend an additional “communications allowance” of £10,000 a year on glossy reports to voters, newsletters, surveys or sophisticated websites. While these cannot be overtly political in nature, they can be used for subliminal campaigning purposes.

Technically, a marginal seat is one where the sitting MP has a small majority. But other factors can affect voting: if the the MP’s local party is neither energetic or harmonious, or an unpopular policy has been pushed through, such as closing a hospital.

Tory targets

M25 corridor
Dartford
Gillingham
Rochester and Strood

Midlands
Milton Keynes North
Wolverhampton South West
NW Leicestershire

Yorkshire
Calder Valley
Colne Valley
York Outer (new seat)

Con/ Lib Dem battlegrounds
Devon Central (new seat)
Meon Valley (new seat)
Romsey & Southampton North

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