It was the moment the “Back Boris” team had dreaded but half-expected – when months of careful campaign planning were thrown off course by revelations about their man’s private life. As votes were being counted on Thursday afternoon in the final ballot of MPs for the Tory party leadership, a member of Johnson’s inner circle was upbeat but still on edge. As he paced the Commons corridors he knew Johnson would come top by a mile and reach the final two candidates – but there was no way he would lower his guard.

Asked how it was all going, he replied: “Yes, well, thanks. But you never know what can happen next, do you?”

With Boris Johnson, no one knows what is round the corner.

In campaigning terms he is ranked a grade one liability. Affairs, gaffes, diplomatic blunders have marked his career at every turn. That is why his advisers have gone to such lengths over recent weeks to keep him off the television, and minimise the opportunities for him to commit indiscretions.

Since the balloting of Conservative MPs began last week, Johnson has been constantly visible in the Commons – but at the same time completely off limits for any informal chat. His team have wanted him “out there” and on show but on a leash and silent, except in meetings with those whose support he has sought, behind closed doors.

Whenever he has conducted Commons walkabouts of late, Johnson has been granted a personal minder shadowing him on his strolls, often in the person of the former Tory MP James Wharton, a key member of his team. On numerous occasions Wharton has been seen physically tugging him away from potential conversations that might be overheard by a member of the press, or, just as dangerous, someone from a rival campaign.

But not even Lynton Crosby, the veteran mastermind of Tory campaigns, who is now advising team Johnson, could have planned for Friday evening’s revelations in the Guardian.

The news that police had been called to the flat Johnson shares with his partner Carrie Symonds, after neighbours had rung 999 having heard a row in the early hours, was a real-life campaign nightmare. Despite repeated requests for comment over several hours on Friday, Team Boris were stumped and had nothing to say at all.

Shortly before the story broke, the Back Boris press operation had been “on grid”, putting out a release of comments he had made to Conservative councillors headlined, somewhat eerily as it turned out, “The Hour is Darkest Before the Dawn”. It carried parts of a speech he had made saying the party had to improve its fortunes after the low ebb of the local and European elections. “We can turn this thing round,” Johnson had told his audience.

Whether the late-night argument damages Johnson and turns out to be his “darkest hour” – and whether he can now “turn things round” and put the campaign back on track – are now the big questions in the battle for No 10.

Too much is unknown about Johnson for any of his supporters to be comfortable. There are issues relating to his private life that are known to worry his campaign team and some of those closest to him, questions about previous relationships, the number of children he has fathered, and even the durability of his current relationship with Symonds. On another front altogether – the serious business of his programme for government – plenty of Johnson backers among MPs suspect his plans on Brexit lack depth and credibility and represent little more than “cake and eat it” rhetoric that could be exposed as such in coming weeks. His plans to cut taxes for the best-off have also caused disquiet among some who support him.

His biographer Sonia Purnell, who worked with Johnson as a journalist in the 1990s, said the next 48 hours could prove crucial in determining whether the ex-foreign secretary remains the hot favourite among Tory members, or whether the contest turns in favour of Jeremy Hunt. Purnell said she was “shocked but not surprised” by the story about his row with Symonds: “I have seen his temper and I do not want to see it again. It is really quite scary.”

A key factor, she told the Observer, would be whether Johnson and Symonds are seen together “all smiles and supportive” in the coming days. Yesterday she did not accompany Johnson at the first hustings with party members in Birmingham, where Johnson refused to answer any questions about their late-night shouting match.

Play Video 3:46 Boris Johnson refuses to answer questions about police visit to his home - video

Purnell says there has been talk among those close to the family that all is not well in the relationship and that Johnson has found it difficult to accept that his marriage with barrister Marina Wheeler, with whom he has four children, is over. Some close to Johnson had worried that he might not be emotionally strong enough to run a leadership campaign, given the recent traumas of his family life.

“There have been suggestions from his circle that he has not fully come to terms or accepted that his marriage is finished,” Purnell said. “There is also the question of the divorce. We have not heard news of that and there is no explanation of why it has not come through yet.”

While Hunt will steer clear of Johnson’s private issues, he can maximise his discomfort subtly in other ways, nonetheless. “If you want the top job you’ve got to turn up for the interviews,” the foreign secretary said on Saturday. Some of Hunt’s supporters felt able to go rather further. Mark Garnier MP, former minister of international trade, who backs Hunt, replied when asked if he thought his man would benefit from Johnson’s local difficulties: “On the one hand you think, this is bollocks, people have arguments. My wife and I have ding dongs every now and then like all couples - although the line ’get off my fucking laptop’ does make you wonder if she was checking his browsing history (I’m just putting that out there).The other side of this is that he has fucked up on the very first evening.

“My good-natured instinct is that we would not want to capitalise on this, of course not. But you have to say this is the type of attention that Boris attracts. He has the capacity to be front page news for this kind of thing. Obviously I know Boris well, and I would be perfectly happy with him being prime minister, I just think Jeremy would be better. I don’t want to get into an argy-bargy about his character. We want to win this on policy.”

On Saturday the pro-Johnson lobby was out in force, arguing that Johnson’s and Symonds’s neighbours who leaked the story of the row to the Guardian were politically motivated and that Tory members in the shires would see that the whole fuss was about nothing more than a red wine induced tiff, whipped up deliberately by “lefties”.

A Tory MP who does not back Johnson said he feared that, as usual, he would ride the story out and that his backers would somehow turn it to his advantage. “If Boris was not acceptable to Conservatives we would know by now. You can never be sure but I don’t think a row with his partner will harm him when no end of far worse things have not.”