David Cameron may be far from certain to win a majority at the election but he is still the first Conservative Party leader since 1987 to enter the campaign with a clear poll lead.

Here are the results of polls conducted just before each of the past five elections was called. All figures are taken from ICM.

12 May 1987

Conservatives 43%

Labour 29%

Liberal Democrats 25%

7 March 1992

Conservatives 39%

Labour 42%

Liberal Democrats 15%

2 March 1997

Conservatives 30%

Labour 48%

Liberal Democrats 16%

22 April 2001

Conservatives 33%

Labour 47%

Liberal Democrats 14%

3 April 2005

Conservatives 34%

Labour 37%

Liberal Democrats 21%

Today's Guardian/ICM poll puts the Tories on 37 per cent, with Labour 4 points behind on 33 per cent.

It's still remarkable to think that while a 3 per cent lead in 2005 (Labour won 35.3 per cent to the Tories' 32.3 per cent of the actual vote) was enough to give Tony Blair an overall majority of 66, a Tory lead of 4 per cent would likely leave Labour as the single largest party in the Commons.

Only twice since the Second World War (in 1951 and in February 1974) has the party with the most votes not won the most seats in parliament.

If the Tories are the winning party in votes but the losing party in seats, we can expect to see a series of newspaper headlines declaring: "Crisis as winning party loses election".

The resultant anger and confusion could transform the dimensions of the debate about electoral reform.

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