Should his private life be held up to our scrutiny?
A row with his girlfriend has focused attention on the murky past of the would-be Prime Minister, but do fellow Tories and the public really care about it, asks Kim Bielenberg?
The former Prime Minister David Cameron once told a story about canvassing with Boris Johnson during an election in the posh London suburb of Hampstead.
Cameron recalled how an "attractive, relatively middle-aged woman" came to the door and declared: "Boris, good to see you. Really lovely to see you. You are the father of one of my children."
Johnson, who was running in the London mayoral election, was nonplussed and burbled: "Cripes! Oh God! Oh cripes!"
According to Cameron's account, Johnson mumbled: "Do the media know? What is it? Is it money?"
She said: "No, you're the father of one of my children. I'm her maths teacher."
Until eight days ago, despite all his well-publicised foibles and scandals, Johnson seemed like a dead cert to become the next Prime Minister - the man who could have the future of the border between Northern Ireland and the Republic in his hands (a frontier he has likened in importance to the border between Camden and Islington).
But now, after details of an apparent blazing row with his girlfriend Carrie Symonds found its way into the press, his character, his past misadventures and suitability for the job are coming under much closer scrutiny. And not a moment too soon.
Police were called in the middle of the night after a neighbour heard screaming and banging from the flat, as Symonds could be heard telling Johnson to "get off me" and "get out of my flat".
Symonds then reportedly told him he had ruined a sofa with red wine and added: "You just don't care for anything because you're spoilt. You have no care for money or anything."
In interviews since the bust-up, Johnson has seemed erratic, evasive and, at times, utterly incoherent. At one stage in an interview on Tuesday, he was asked about his hobbies - and rambled on about how he liked to relax by making model buses out of wine crates.
One was left wondering if these wine-crate buses were merely a flight of fancy, or real artefacts, adorning his girlfriend's flat, ready to be chopped to pieces when he flies into the next rage.
The reporting of the domestic row between the putative Prime Minister and his girlfriend raises questions about whether the private behaviour of a politician should disqualify him from high office.
It could be argued that if harmony in their personal relationships and fidelity were to be the yardsticks by which we measure politicians, the corridors of power would be denuded of considerable talent.
There would be no room for Bill Clinton, John F Kennedy and Franklin Delano Roosevelt, just for starters.
In Britain and America, a straightforward affair, without too many bells and whistles, no longer seems to disqualify a politician from high office.
Donald Trump has set the bar low for the standards of decorum expected of a modern politician. He managed to win an election even after a film emerged where he appeared to boast in crude terms about groping women. If that doesn't disqualify you from high office, there must be hope for every sex pest lurking around the next street corner.
Trump boasted during his presidential campaign: "I could stand in the middle of 5th Avenue and shoot somebody and I wouldn't lose voters." And who are we to disbelieve him?
While the odd affair may be overlooked, does there not come a point when the character of the candidate does become relevant?
Is it possible to draw precise boundaries between a public and a private life?
If a man cannot be trusted by his wives, his girlfriends, his friends and his family, why should the public be inclined to trust him?
Johnson is believed to have six children, including four from his second wife Marina Wheeler, and two children from extra-marital affairs.
According to his biographer, Sonia Purnell, his attitude to women - endless affairs leaving a string of women, and at least one pregnancy termination, behind him - has long been one of entitlement and lack of respect.
In 2012, when he was Mayor of London, a High Court judge ruled that a newspaper was justified in publishing stories about him fathering a child, because his "recklessness" in conducting extramarital affairs, which has resulted in two children being born, called into question his fitness for public office.
It is the general picture of his character - not just the affairs - that has led many of his acquaintances to suggest that he is unfit to be Prime Minister.
In assessing Johnson's personality, Max Hastings, his former boss when he was a reporter on the Daily Telegraph, quoted the Irish philosopher Bishop Berkeley: "It is impossible that a man who is false to his friends and neighbours should be true to the public."
Hastings suggested this week that Johnson would not recognise truth - whether about his private or political life - if confronted by it in an identity parade.
He was sacked as a young reporter from The Times for inventing a quote and fired from the Tory front bench for lying to the Conservative leader Michael Howard about one of his affairs.
Early in his career, he was also taped in a conversation as his old school friend, Darius Guppy, discussed plans to have a journalist beaten up and Johnson talked of helping Guppy to get the reporter's contact details.
Boris may have many detractors, including the BBC presenter who called him a "nasty piece of work", but it remains to be seen if an increased focus on his murky past deters the clearly smitten Tory members, who will choose the next Prime Minister.
If nothing else, he has won the support of a man, who is now frequently compared to him. Donald Trump says: "I think Boris would do a very good job.
"I think he would be excellent."