The last time it happened was more than 30 years ago.
If the Conservatives were to lose both of the by-elections taking place this Thursday – in Wakefield and in Tiverton & Honiton – it would be only the seventh time a government has suffered such a double defeat since the Second World War.
Wakefield in West Yorkshire was won by the Tories from Labour at the 2019 general election, but Labour is now hoping to take it back.
To overturn the Conservative majority of 3,358, Labour needs a swing in the share of the vote of 3.8 percentage points – in other words, four in every 100 people who voted Tory in 2019 would need to switch to Labour.
A far bigger swing is required for the Tories to lose the seat of Tiverton & Honiton in Devon.
The Liberal Democrats are the main challenger in this constituency, but it would take a swing of 22.8 points for them to win, or the equivalent of 23 in every 100 people who voted Conservative in 2019 switching directly to the Lib Dems.
This is a smaller change than the one achieved by the Lib Dems in the North Shropshire by-election in December last year, when they took the seat from the Conservatives on a 34.1-point swing.
It would also be smaller than the 25.2-point swing in the Chesham & Amersham by-election in June 2021, which again saw the Liberal Democrats take the seat from the Tories.
If the Lib Dems manage a swing in Tiverton & Honiton of more than 25.4 points, it would rank as one of the 10 largest swings against a government since 1945 in a by-election which saw a change in both party and MP.
The biggest swing of this kind took place in July 1993 at the Christchurch by-election, which was won by the Lib Dems on a 35.4 point swing from the Conservatives.
The Tiverton & Honiton seat has been held by the Tories since its creation in 1997.
MP Neil Parish won a majority of 24,239 at the 2019 general election, but resigned earlier this year after admitting watching pornography in parliament.
Wakefield had been a Labour seat continuously from 1932 until 2019, when it was won for the Conservatives by Imran Ahmad Khan.
Khan resigned earlier this year after being convicted for sexually assaulting a boy.
Polling stations for the by-elections in Wakefield and Tiverton & Honiton will be open from 7am to 10pm on Thursday, with the results expected between 4am and 6am on Friday.
If the Conservatives do lose both by-elections, history might provide some comfort.
The last time the Tories suffered such a double defeat while in power – in November 1991 – the party went on to regain both seats just five months later at the 1992 general election.
As well as November 1991, the other occasions since the Second World War when a government has lost two by-elections on the same day were July 1973, March 1968, November 1967, September 1967 and November 1962.