Labour currently has a lead of about 20 points in the opinion polls, whereas the Conservatives won the last election by an 8-point margin. Such a difference in the real poll would mean that 14 per cent of voters had changed sides. This is what what the pundits mean by "swing" - and as you can see from the swingometer a 14 per cent swing would give Labour the Norfolk seat of Gillian Shephard, the Education Secretary.
The idea of a uniform swing is less useful when it comes to the Liberal Democrats and other parties. The Lib Dems in particular are good at concentrating their votes where they matter. And if the Tory vote falls as much as some opinion polls suggest, about 40 seats become vulnerable to them.
Most seats have been redrawn since 1992. The effect has been to increase the size of the House of Commons by eight seats from 651 to 659. So a party needs 330 seats for an overall majority of one.
But the political effect has been small, despite the fact that redrawing was supposed to give growing Tory suburbs more MPs. The system is still biased against the Tories, who would be 33 seats behind Labour if both parties won equal shares of the national vote. John Major would have had a majority of 27 instead of 21, on 343 seats (up seven) to Labour's 273 (up two) and the Liberal Democrats' 18 (down two). This is the baseline for this election.
Seats which changed hands in a by-election are treated as belonging to the party which won in 1992.
Boundary changes also mean that some seats have changed hands. Slough and Swindon North have Tory MPs although under new boundaries they would have been Labour last time.
And Ilford South, Warrington South and Kingswood will count as Labour "gains" if they are won, even though they already have Labour MPs.
Estimates of how new seats would have voted in 1992 are from Colin Rallings and Michael Thrasher's Media Guide to the New Parliamentary Constituencies, published by the BBC, ITN, PA and Sky.
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