There are not many prime ministerial careers that do not, eventually, end in failure. Fewer still recover from the kind of self-inflicted wounds Boris Johnson has given himself.
Politicians can survive scandals and even overcome multiple calamities across a career. At some point, however, the improprieties and foul-ups congeal into an unrepairable mess, dragging their unhappy bearer into the abyss.
This is the tipping point - the moment that, as Dr Mark Garnett, author of The British Prime Minister in an Age of Upheaval, puts it: “The gilt comes off the gingerbread man.”
If Mr Johnson falls in the next few weeks, there are few historical comparisons that will do justice to the precipitous nature of his decline.
This is the man, after all, who took a parliament paralysed for over a year with a set of seemingly irreparably divided Conservative MPs to an 80-seat majority.
Perhaps the closest example is Anthony Eden. As successor to Winston Churchill, he called the 1955 snap election and increased the Conservative majority from 17 to 60. Yet he was out of office in under two years, having presided over the Suez Crisis - the final, humiliating confirmation that Britain was no longer a great power.
Mr Johnson, though, is no Sir Anthony. Whatever the wrongs of Downing Street’s prolific partying, it does not quite compare to colluding with France and Israel to create the conditions for a fake peacekeeping operation aimed at toppling the Egyptian government.
Despite just two years in office, Mr Johnson’s influence on Britain, unlike Sir Anthony’s, is likely to last for decades.
Not only was he Prime Minister “at the time of the most significant health crisis in Britain for 100 years” but, by forcing through a hard Brexit and transforming the Conservative Party, he “will have shaped British history, its economy and foreign policy, for the entire middle 21st century”, said Sir Anthony Seldon, author of The Impossible Office? and a biographer of multiple recent premiers.
Of course, Mr Johnson may yet cling on to the keys of Downing Street for months, perhaps even years. The real question is whether he has passed the tipping point from which prime ministers do not recover, no matter how long they stay in office.
Rather than Sir Anthony Eden, Dr Garnett offers up Sir John Major as the best comparison. While a steady flow of sleaze allegations damage Sir John’s electoral hopes, the real blow to his popular support came just five months after his 1992 election victory, when Britain crashed out of the Exchange Rate Mechanism on Black Wednesday.
“He went from hero to zero overnight,” points out Dr Garnett, with public confidence in the Conservatives never recovering. The difference then was that, with years to go until the next election, Tory MPs were not thinking: “My God, I’m going to lose my seat.” Today’s Conservative MPs do not have that luxury.
As with all historical parallels, this one is far from perfect. For Sir John, his whole party was tarnished by Black Wednesday, but leadership hopefuls such as Rishi Sunak might hope to escape any flack for Number 10’s partying.
Moreover, as Dr Garnett points out, Black Wednesday, like Suez, was a single catastrophic event. Mr Johnson has been undermined by a series of scandals.
“Everyone seems to have forgotten about the Owen Paterson debacle, but only because there have been so many other crises since,” says Dr Garnett.
In that sense, he is more like Margaret Thatcher, who suffered a number of popularity-sapping fiascos before the Eastbourne by-election tipped her over the edge in 1990.
The key point however is that as with Sir John, Mr Johnson may have reached the point where public opinion has solidified and, no matter what he does, it cannot be changed.
“Once you feel that the public mood has turned, you start doing things which are transparently calculated to restore your fortunes,” said Dr Garnett.
“But if people have decided that you shouldn't really be prime minister anymore, then that kind of gambit is doomed before it starts. Major’s Back to Basics campaign, for example, was a colossal disaster.
If Mr Johnson is indeed that toxic - and the latest polling would suggest that he is - then it would be in the party’s interests to defenestrate him as soon as possible.
The Prime MInister has forged a Conservative coalition others probably could not have done, but it has proved harder to keep together than he might have hoped.
As Sir Anthony Seldon points out, the Prime Minister has struggled to play two very different roles - the disruptive force that broke the Brexit stalemate and the national healer who brought the nation together in its aftermath.
“Macmillan put the lid back on the extraordinarily volatile position in Britain, with the country deeply divided after Suez,” he said. “It’s been much harder to put the cap back on after Brexit.”
Rather than forging a new politics from the chaos of 2017 to 2019, Mr Johnson may simply have been a temporary release from it.