If the Conservatives are beaten at the polls in both Mid-Bedfordshire and Tamworth on Thursday, it would mean 10 seats have changed hands at parliamentary by-elections in the past three years – eight of them being defeats for the Government.

This is a level of churn not seen since the 1990s, when John Major was prime minister and the Tories had clocked up almost 18 years in power at Westminster.

The two seats up for grabs this week were won comfortably by the Conservatives at the 2019 general election and will need large shifts in public opinion to change hands.

But the swing in the share of the vote that would spell defeat for the Tories is similar in size to the swings achieved by Labour and the Liberal Democrats at recent by-elections.

– Tamworth

Tamworth was created at the 1997 general election and held by Labour for 13 years before being gained by the Conservatives in 2010.

The constituency’s former MP, Chris Pincher, increased the Tories’ share of the vote at each subsequent election, from 46% in 2010 to 50% in 2015, 61% in 2017 and 66% in 2019.

He also pushed up the Conservatives’ majority from 6,090 in 2010 to 19,634 in 2019.

Labour finished in a distant second place in 2019 on 24% of the vote, with the Liberal Democrats even further behind on 5% and the Greens on 2%.

To win the seat on Thursday, Labour needs a swing in the share of the vote of 21.4 percentage points.

In other words, a net change of 22 in every 100 people who voted Conservative in 2019 needs to switch sides to Labour.

Swings of 20 points or higher used to be rare at parliamentary by-elections but there have been no fewer than six of them in the last three years.

The most recent example was earlier this month, when Labour won Rutherglen & Hamilton West from the SNP on a swing of 20.4 percentage points.

In July, Labour took Selby & Ainsty from the Conservatives on a swing of 23.7 points, while the Liberal Democrats gained Somerton & Frome, also from the Tories, on an even bigger swing of 29.0 points.

The other three examples have all involved the Lib Dems taking seats from the Conservatives, and all 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 by-election in Tamworth also has an intriguing parallel in recent history.

Before 1997, this area of the country was represented by the constituency of South East Staffordshire.

A by-election took place in South East Staffordshire in April 1996, roughly one year ahead of the general election.

Then, as now, the Conservatives went into the contest defending a large majority – only to see Labour win the seat on a huge swing of 22.1 percentage points.

– Mid-Bedfordshire

The other by-election taking place on Thursday appears to be less of a two-way fight than the poll in Tamworth.

Labour and the Liberal Democrats are both claiming to be the main challenger to the Conservatives in Mid-Bedfordshire.

The seat has been in existence since 1918 and has elected Conservative MPs continuously since 1931.

Former MP Nadine Dorries held the seat from 2005 until her resignation earlier this year, winning 60% of the vote at the 2019 general election.

Labour finished second in 2019 with 22% of the vote and the Lib Dems were third on 13%, with the Greens on 4%.

To win Mid-Bedfordshire, Labour needs a swing in the share of the vote of 19.1 points – slightly smaller than the one needed in Tamworth.

The Liberal Democrats would need an even bigger swing of 23.6 points to take the seat.

Recent events have shown the parties can pull off swings of such a size, but if it is a close contest and the anti-Conservative vote divides between Labour and the Lib Dems, the Tories could hold the seat.

Labour will make history if it pulls off a win because it would be the largest Conservative majority (24,664) overturned by the party at a by-election since 1945.