This coming general election, the Conservative Party has to claw back significant ground and the main fight will be in the marginal seats.
A marginal seat is a district or constituency held with a particularly small majority. These seats require a smaller swing to change hands and are therefore typically the focus of most of the parties' campaign resources.
The reason is simple. The UK electoral system is structured in such a way that the election’s outcome, both in terms of which party forms the government and the size of that party's parliamentary majority over all other parties, is determined in about 20% of constituencies in the marginal seats.
So in theory, a winning party could lose the national vote, but win more seats because it performs better in the marginal constituencies than it does in the country as a whole.
For the Tories to cut into Labour's huge parliamentary majority, it needs to take away the most vulnerable 50 Labour-held marginals, where the present MP’s majority is less than 10% over the Conservative challenger.
And the ground is there to be made up. In these constituencies, Labour's share of the vote has fallen from 45% in 2001 to 37% now. However, there has also been a fall in the Conservative share in these seats - down seven percentage points to 32% now.
This represents a swing to the Conservatives from Labour of 0.5% (lower than their 2.7% swing nationally), and only enough for Michael Howard to claim four of the 50 Labour seats, the Liberal Democrats to gain 2 and the SNP 1.
Clear pattern
Underlying these figures is a very clear pattern. In virtually all the marginal seats, the Conservative share of the vote has hardly moved since 2001. In the most marginal seats the party is defending from the Labour Party, its share of the vote is currently 43%. In 2001 it was 45%.
The pro-hunting lobby is expected to target marginal seats during the election campaign, and one of the trade unions may focus on a constituency where lots of jobs had been moved offshore – for example, by relocating a call centre to India.
Lobby groups know the significance of marginal seats. In March, Muslim leaders instructed Muslims to vote against Labour in nearly 40 marginal constituencies in protest at the war in Iraq and the government’s detention policy.
They have targeted seats with a large number of Asian voters including those held by Jack Straw, the foreign secretary, in Blackburn, Margaret Beckett, the environment secretary, in Derby South and Stephen Timms, the energy minister, in East Ham.
Opposition parties will hope to exploit the volatile nature of the marginal seats.