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Swing in a British political context is a mathematical calculation which allows the results of two constituencies to be compared.
The term "swing" has a different meaning in Australia, which has a different voting system. See Swing (Australian politics).
In Britain, the conventional swing, or Butler Swing is defined as the average of the Conservative % gain and Labour % loss between two elections, with the percentages being calculated on the basis of the total number of votes (including those cast for candidates other than Conservative or Labour). There is an alternative version called Steed Swing which calculates the percentages on the basis of votes cast for Conservative and Labour only. It is possible for the same election to have a Butler Swing to one party and a Steed Swing to another.
As an example, if in the previous election Labour had 45%, the Conservatives 35% and the Liberal Democrats 20%, and in the new election the Conservatives had 45%, Labour had 40% and the Liberal Democrats 15%, then the swing would be the average of the Conservative gain (10%) and Labour loss (5%), which makes +7.5%.
Swing was originated by David Butler, a political science academic at Nuffield College, Oxford. In a contribution to 'The British General Election of 1945' he wrote "this measurement of 'swing', admittedly imperfect, does give us a broad idea of the movement of opinion from Conservative to Labour" and went on to compare the swings in each area of the country. The concept became important in the general elections of the 1950s when it was found that there was a relatively uniform swing across all constituencies. This made it easy to predict the final outcomes of general elections when few actual results were known, as the swing in the first constituency to declare could be applied to every seat. Taking the national vote shares in an opinion poll could also easily be translated into likely seat outcomes. Election night television programmes from 1955 have usually featured a device known as the 'swingometer' which consisted of a pendulum which could point to the swing nationally and illustrate the outcome.
However, the immediate post-war period in British politics was noted for the strength of the two-party system and the weakness of the Liberal Party. The calculation of swing did not make any allowance for other parties and when the votes for other parties rose, demands arose for a more sophisticated measurement. Simply substituting the Liberal Party for the Labour Party in the calculation could provide a measure of a 'Swing between Conservative and Liberal', although election results showed that this was not a useful predictor in seats which were being fought by these parties. Nevertheless these calculations, and others involving parties such as the Nationalist parties in Scotland and Wales, can be seen in some published analyses of elections.
It should be noted that swings of over 10% in magnitude are very rare. Taking British politics after 1945 exclusively (as that election occurred ten years after its predecessor, and in a completely different political climate), only the 1997 general election had a national swing of more than 10% in magnitude, and that was -10.23%. The table below shows the national swing across Great Britain, and the number of individual constituencies out of more than 600 which had a swing of over 10% in magnitude.
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