Labour’s victory in Rutherglen & Hamilton West marks the eighth time a Westminster parliamentary seat has changed hands at a by-election since the 2019 general election – and the sixth to do so on a huge swing of more than 20 percentage points.
To win Rutherglen from the SNP, Labour needed a swing in the share of the vote of 4.9 percentage points.
In other words, a net change of five in every 100 people who voted SNP in 2019 needed to switch sides to Labour.
Labour ended up achieving a swing of 20.4 points – more than enough to win the seat comfortably.
On the same day in July, the Liberal Democrats won Somerton & Frome from the Tories on an even bigger swing of 29.0 points.
The Lib Dems have made three other gains from the Conservatives since the 2019 general election, all of them on enormous swings: Tiverton & Honiton in June 2022 (29.9 percentage points), North Shropshire in December 2021 (34.1 points) and Chesham & Amersham in June 2021 (25.2 points).
While swings of this magnitude are not unheard of at by-elections, it is unusual to have so many in such a short space of time.
The Liberal Democrats took two seats from Labour on swings of over 20 points at by-elections in the early noughties: Brent East in September 2003 and Leicester South in July 2004.
There were four by-elections during the 1992-97 Conservative government which saw swings away from the Tories of more than 20 points: Newbury in May 1993 (won by the Lib Dems), Christchurch in July 1993 (also won by the Lib Dems), Dudley West in December 1994 (won by Labour) and South East Staffordshire in April 1996 (also won by Labour).
To have six by-elections with swings of more than 20 points within the space of three years, though, is unprecedented in modern history.
Another swing of more than 20 percentage points could be on the cards later this month, should Labour manage to win the Tamworth by-election on October 19.
The party needs a swing of at least 21.4 points to take the seat from the Conservatives.
Also taking place on October 19 is a by-election in Mid-Bedfordshire, which Labour would need a swing of 19.1 points to win from the Tories.
The Liberal Democrats are also targeting Mid-Bedfordshire and would need an even bigger swing of 23.6 points to win.
Recent events have shown the parties can pull off swings of such a size – though the outcome in Mid-Bedfordshire may be affected by Labour and the Lib Dems both claiming to be the main challenger to the Conservatives, rather than it being a clear two-horse race.
The 23.7-point swing at Selby & Ainsty in July was the largest achieved by Labour at a by-election since 1994 and its second largest since 1945 – the largest being the swing of 29.1 points at Dudley West in December 1994.
Rutherglen also ranks as one of the largest-ever swings at a Westminster by-election in Scotland where a seat has changed hands.
Only four such contests since 1945 have seen larger swings, the most recent being at Glasgow East in July 2008, which the SNP won from Labour on a 22.5-point swing.
Two other Westminster seats have changed hands at by-elections since the last general election: Wakefield, which Labour gained from the Conservatives in June 2022 on a swing of 12.7 points, and Hartlepool, which the Tories won from Labour in June 2021 on a swing of 16.0 points.
Labour had hoped to gain Uxbridge & South Ruislip from the Conservatives at the by-election in July of this year, needing only a swing of 7.6 points to do so.
In the event the party managed only a 6.7 point swing and the Tories held the seat.