The Liberal Democrats claim the result has sent a “shockwave through British politics” by showing that seats in the “Blue Wall” of Tory seats in southern England are vulnerable.
This remains to be seen – they won just 11 seats at the last election and have effectively been in a fight for survival. This win keeps in the game – for now – and no more.
But the warning signs were there for Johnson in the local elections in May where the Tories lost councillors in places including Tunbridge Wells, Surrey and Oxfordshire, as well as the West of England mayoralty.
Nothing as spectacular as the loss of Chesham and Amersham – a seat which had been in Tory hands since its creation in 1974 and which returned a Tory MP with 55 per cent of the vote in 2019.
The lingering Brexit divide, the Prime Minister’s style of politics and demographic shifts which have seen younger, liberal voters priced out of cities and head to the suburbs and commuter belt have all contributed to an erosion of previously rock-solid Tory heartlands.
The shift is nowhere near as significant as the collapse in Labour support across former industrial areas in the north and Midlands - the crumbling “Red Wall” which helped Mr Johnson secure his 2019 landslide.
But the Prime Minister’s focus on “levelling up” means Tory priorities may lie beyond their southern strongholds, while the Chesham and Amersham contest showed that proposed planning reforms aimed at boosting housebuilding may not be vote-winners in the home counties.
After confirmation of Sarah Green’s victory, defeated Tory Peter Fleet hinted that the Conservative effort in Chesham and Amersham had not been able to match the resources directed at the Buckinghamshire seat by the Liberal Democrats - another issue to ponder in Conservative Campaign Headquarters (CCHQ).
The Lib Dems “didn’t just throw the kitchen sink at this constituency, I think it was the microwave, the table, the oven, the dishwasher, the dog, the cat and anything else that was lying around as well”, he said.
“And we should consider that when we reflect upon the extraordinary nature of the result.”
CCHQ sought to play down the wider significance of the result, with a source insisting “by-elections are always difficult for the governing party, particularly eleven years into government” - although this did not stop the Tories winning Hartlepool in May.
But the source acknowledged “there is no getting away from the fact this is a very disappointing result”.
For the Lib Dems, the result offers hope after a series of poor general election results since the end of the coalition government.
And they exploited this opportunism. Dame Cheryl Gillan, whose death prompted the by-election, was a staunch opponent of HS2 which runs through the constituency.
The Lib Dems campaigned against HS2 in this seat – but remain in favour of high-speed rail nationally. Party leader Sir Ed Davey has yet to explain this discrepancy.
More significantly was how Labour came fourth, behind the Greens, with just 622 votes and lost its deposit. It is said to be the party’s worst by-election performance ever.
It’s an ominous result ahead of the Batley and Spen by-election on July 1 as Labour look to see off a Tory challenge. Defeat for Labour, so soon after the loss of Hartlepool, could see party leader Sir Keir Starmer coming under renewed pressure.
And there’s strong advice for him from Justine Greening, a former Education Secretary. Writing in The Yorkshire Post this weekend, she will say: “Mr Starmer could lead a wider, national debate on levelling up.
“The by-election result in Chesham and Amersham shows no party can take voters for granted as the realignment of British politics continues apace.
“Instead it’s the Lib Dems tapping into voter concerns and winning in Chesham and Amersham whilst the Labour vote collapses there too.”
By-elections invariably do not translate into general election results. They’re one-offs. But they’re about momentum. The Tories had it, the Lib Dems have it and Labour, the main party of Opposition, do not. That’s the key lesson.