Swing (politics)

An electoral swing analysis (or swing) shows the extent of change in voter support, typically from one election to another, expressed as a positive or negative percentage. A multi-party swing is an indicator of a change in the electorate's preference between candidates or parties (e.g. Conservative to Social Democrat). A swing can be calculated for the electorate as a whole, for a given electoral district or for a particular demographic.

A swing is particularly useful for analysing change in voter support over time, or as a tool for predicting the outcome of elections in constituency-based systems. Swing is also usefully deployed when analysing the shift in voter intentions revealed by (political) opinion polls or to compare polls concisely which may rely on differing samples and on markedly different swings and therefore predict extraneous results.

A swing is calculated by comparing the percentage of the vote in a particular election to the percentage of the vote belonging to the same party or candidate at the previous election.

One-party swing (in percentage points) = Percentage of vote (current election) − percentage of vote (previous election).

Examples include the comparison between the 2006 and 2007 Ukrainian Parliamentary elections and the win of AAP in the 2015 Delhi elections.

The above charts show the change in voter support for each of the six major political parties by electoral district and nationwide vote results.

In many nation states' media, including in Australia and the United Kingdom, swing is normally expressed in terms of two parties. This practice is most useful where most governments tend to be from an existing two-party system but other candidates do sometimes run, and is used to predict the outcome of elections in constituency-based systems where different seats are held with different previous levels of support. An assumption underlies extrapolated national calculations: that all districts will experience the same swing as shown in a poll or in a place's results. The advantage of this swing is the fact that the loss of support for one party will in most cases be accompanied by smaller or bigger gain in support for the other, but both figures are averaged into one. Employing the two assumptions allows the analyst to compute an electoral pendulum, predicting how many (and which) seats will change hands given a particular swing, and what size uniform swing would therefore bring about a change of government.

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