Electoral swing is a way of describing how support has moved between parties from one election to another. It is usually expressed in percentage points.
A simple vote-share change
If a party’s vote share rises from 30% to 36%, its share has increased by six percentage points. That is a change in vote share, but the word “swing” is often used more specifically to compare movement between two parties.
Two-party swing
The traditional Butler calculation takes the average of one party’s percentage-point gain and the other party’s percentage-point loss.
For example, if Party A rises by 6 points while Party B falls by 4 points, the two-party swing from B to A is 5 percentage points: (6 + 4) ÷ 2.
Why swing is useful
It provides a compact way to compare changes between elections and between constituencies. Historically it has been particularly useful when two large parties dominated a contest.
Why swing can mislead
Modern British elections are often multi-party contests. Support can move between several parties simultaneously, so a single two-party swing may hide important changes elsewhere.
It is also possible for national movement to be distributed unevenly. A party might rise sharply in one region and barely move in another.
What is uniform national swing?
Uniform national swing applies the same assumed movement in party support to every constituency. It can be useful as a simple illustration, but real elections are not uniform. Local and regional differences mean it should not be treated as a precise seat forecast.