There are times when a government loses its authority in such a hugely symbolic way that a sea change which will sweep it away in an election defeat becomes inevitable.
Rubbish piled up in the streets during the 1978-79 “winter of discontent” was an image that haunted Labour long after Margaret Thatcher’s 1979 election victory. As James Callaghan, the Labour prime minister she defeated, admitted to an aide before the election: “There are times, perhaps once every 30 years, when there is a sea change in politics. It then does not matter what you say or what you do. There is a shift in what the public wants and what it approves of.”
Sometimes, it’s personal: Thatcher stubbornly sticking to her hated poll tax spelt the end of her premiership. Her successor John Major and the Tories never recovered their economic credibility after Black Wednesday, when Britain was booted out of the European exchange rate mechanism. An inevitable defeat at the following election brought an end to 18 years of Tory rule.