IT WAS “not the result I was expecting,” admitted Peter Fleet, a former motor-industry executive, in his concession speech in the small hours of June 18th. He thought he was about to become the next Conservative MP for Chesham and Amersham, which until that night had been among the party’s most dependable seats.

Since its creation in 1974, the Conservatives have never won less than half of the vote there. Dame Cheryl Gillan, the MP whose death in April triggered the by-election, secured a majority of more than 16,000 votes and 55% of the total at the general election in 2019. That dominance has come to a stunning end with the victory of Sarah Green, the Liberal Democrats’ candidate, who polled 57% of the vote, on a turnout, decent by by-election standards, of 52%. It follows the Tories’ own dramatic by-election triumph in May, when they seized the north-eastern port town of Hartlepool, a Labour stronghold. Paired with last month’s local elections, it seems that the Conservatives’ march through towns across the north of England and the Midlands may be coming at the expense of some of its older, wealthier southern heartlands.

The Lib Dems’ win, Mr Fleet noted correctly if ruefully, is “absolutely extraordinary”. The market towns of Chesham and Amersham, and surrounding villages with names like Great Missenden, Chalfont St Giles and Buckland Common, are deep Tory England. Connected to the capital by the last two stops of the Metropolitan Line railway, the constituency is filled with affluent professionals, making it the third-least deprived in England, according to government indices that account for education, health, employment and crime. Stiff competition for the local grammar school means star tutors are in high demand.

The swing to the Lib Dems, 25.1 percentage points, is the party’s third-biggest against the Tories. The top two took place in 1993, when the Conservatives had cratered in the polls after Black Wednesday (the pound’s spectacular crash and exit from the European exchange-rate mechanism the year before). Curiously, the triumph in Chesham and Amersham has come when Boris Johnson is riding high in the polls, buoyed by a highly successful vaccine roll-out.

Mr Johnson brushed off the defeat as the product of “particular circumstances”. The Liberal Democrats are fearsome local campaigners, and have a long record of snatching seats in by-elections, only to lose them again in general elections. The electors of Chesham and Amersham have been angered by HS2, a high-speed rail link that punches through the bucolic Chiltern Hills many moved there to enjoy. Mr Johnson’s proposed planning reforms, which will make it easier for developers to build new houses, have also left residents worried. This result will strengthen a revolt among Tory backbenchers against those plans; Mr Johnson said he would push on.

But there is evidence of a deeper malaise in the shires and suburbs that were once firmly Tory. Discontent over HS2 has been brewing for a decade, without troubling the party locally (Dame Cheryl was a firm opponent). Mr Johnson has made clear that his priority is serving his new supporters in poorer regions. Planning reform and HS2 are part of this; so are cuts to foreign aid, a hard Brexit and skirmishes over culture that go down less well in more liberal Tory areas. Ms Green declared that her new constituents had been “taken for granted”.

Anti-Tory tactical voting helped her. Labour, which won 7,166 votes (12.9%) in 2019, saw its vote collapse to just 622. Its 1.6% share was its lowest in a by-election since the second world war. Expect calls for a “progressive alliance” of centre-left parties, recently floated by Tony Blair, the last Labour leader to win a general election, to grow—especially if Labour loses Batley and Spen, another northern seat, in a by-election on July 1st.

Some new, northern Tory MPs see a trickle of losses in the south as a small price to pay. One, representing an old mining area, says the result helps show that “we are not the party of the upper middle classes any more”. But if this shift continues, their older colleagues will have good reason to be worried. In 2019 the Liberal Democrats came second in 80 seats won by the Conservatives—and in 42 of them by a smaller margin than in Chesham and Amersham. Many of these, like Chesham and Amersham, contain affluent suburban commuter towns with lots of graduates. Labour may also gain in such places. Among the Tories vulnerable to the Lib Dems is Dominic Raab, the foreign secretary, in Surrey. On London’s northern fringes Sir Iain Duncan Smith, a former leader of the party, is at risk from Labour.

Such a trend would also suggest a fundamental shift in British politics. Labour’s losses in its former northern heartlands has left it dwelling on its fate in increasingly existential terms. Now the Conservatives must grapple with whether they can stack their new “Red Wall” on top of their southern blue one. The resulting structure may be wobbly.