Political Editor Chris Fisher looks at the election landscape in Norfolk.

Political Editor Chris Fisher looks at the election landscape in Norfolk.

To win the election, the Tories need a shift of support to them that been achieved only once by either of the two main parties - by Tony Blair's New Labour in 1997 - since the second world war.

Just to get a Commons majority of one, they need to gain 116 Commons seats from other parties. And to do that, they will need a swing towards them of 6pc or more.

This makes Waveney (including Lowestoft) one of the key seats in the land. Taking account of slight boundary changes, the Conservatives will need exactly a 6pc swing to unseat Labour's Bob Blizzard, the deputy minister for the eastern region. And if there were a swing to them by that amount from Labour in that party's marginal seats and by the same amount from the Lib Dems in theirs, Waveney would be the 116th and last seat to turn blue.

It would of course be astonishing if there were simultaneously a swing exactly of that amount to the Tories from Labour and the Lib Dems. If the Conservatives were wholly dependent on a swing from Labour it would need to be one of almost 7pc - and Waveney would certainly not then be the crucial seat.

To simplify all of this, if the Conservatives cannot take Waveney, it's unlikely that they will be able to form a government supported by a Commons majority of their own. If, on the other hand, they can overturn Mr Blizzard's majority of 5915 in 2005 and replace it with one of, say, 2000 of their own, they will be very much in business.

Neighbouring Gt Yarmouth will also be a seat to watch. The Tories have to win it, and thereby destroy Tony Wright's Labour majority of 3055. That would require a swing of only 3.7pc, and if they cannot do that, they are going to finish at least 60 seats short of a Commons majority and 25 or more seats short of the Labour total.

In differing ways, both Norwich South and Norwich North should also prove fascinating. The former, indeed, could be one of the most interesting in the whole country.

Former home secretary Charles Clarke had a modest majority of 3653 in 2005, and it has been calculated that it would have been only 3023 if the constituency's boundaries now had existed then. The Liberal Democrats came second last time, with the Tories a further 3000 votes, approximately, behind them.

Since then, the Greens have done well in the seat in elections for the European Parliament and the local councils, and the constituency is being talked up by some as an extremely rare example of a four-party marginal. Mr Clarke will hope to survive by name recognition, by being seen as anti-Brown though Labour, and by the intense local rivalry between the Lib Dems and the Greens.

In Norwich North, Chloe Smith is in the extremely unusual position of defending for the Tories a seat in which there was a Labour majority (for Ian Gibson) of 5459 in 2005. Drayton and Taverham have now been taken out of the seat, and it has been calculated that had the new boundaries existed five years ago, Dr Gibson's majority would have been 6769. But in the by-election last year (fought on the old constituency boundaries) Ms Smith had a majority, over Labour, of 7348.

Norfolk has an extra, ninth, seat of Broadland in this election. It has been calculated that had it been part of the 2005 election, the Tories would have had a majority of 6573 (over the Lib Dems). So it ought to be safe for Keith Simpson, who has moved from the old Mid Norfolk seat that he has represented since 1997. The new Mid Norfolk, which is very different from its namesake, had a 'notional' Conservative majority of 7793 in 2005, and should be won by the party's candidate, George Freeman.

The other Norfolk seats - Norfolk North, Norfolk South, Norfolk SW and Norfolk NW - can also be expected to remain with the party of the MP now representing them.

With boundary changes, Norman Lamb's 10,606 majority for the Lib Dems in North Norfolk in 2005 has been trimmed to 8575. Tories Henry Bellingham and Richard Bacon will be defending adjusted majorities of 8417 and 6719 in Norfolk NW and Norfolk South, and new Conservative candidate Elizabeth Truss inherits a modified majority of 6817 from Christopher Fraser in Norfolk SW.

It don't mean a thing if it ain't got that swing.

'Swing' will be a greatly used word over the coming month. And the concept will underlie all the calculations translating opinion poll findings into Commons seats. But what is it? And how do you work it out?

Swing measures the movement of voter support from one party to another.

At the last general election, Labour won 35.2pc of the votes in the UK, the Tories 32.3pc and the Lib Dems 22.1pc.

Let us assume just for the purpose of this exercise that the voting shares on May 6 will shift to: Labour 31.2, Tories 37.3, Lib Dems 23.1. That would mean that Labour had lost four points, the Tories had gained five and the Lib Dems had gained one (which would also mean that the 'others' had lost two points).

In that event, there would be a 4.5pc swing from Labour to the Tories - enough for a hung parliament, but not enough for a Tory majority in the Commons.

How do you get the 4.5pc figure? You add the Tory gain of five to the Labour loss of four and divide by two.

In this example, there would simultaneously have been a 2.5pc swing from Labour to the Lib Dems (four plus one, divided by two). And there would also have been - much less obviously - a swing from the Lib Dems to the Tories of 2pc (five minus one, divided by two).

Another way of looking at swing is this: If five voters out of 100 switch from Labour to the Tories, it's a 5pc swing to the latter.