When Boris Johnson was in contact with Steve Bannon in July, the message Donald Trump’s ex-chief strategist had for him was straightforward: man up. Step up to the plate. Get off your knees. You can imagine this unpleasant challenge to Johnson’s virility by the self-appointed guru of the populist right. The apparent response has been no more pleasant to behold. Last month, the former foreign secretary was widely – and justly – condemned for writing that a Muslim woman wearing the niqab resembles “a bank robber” and that it is “absolutely ridiculous that people should choose to go around looking like letter boxes”.

In today’s Mail on Sunday he resumed his reckless use of political language, comparing Britain’s negotiating position with the EU on the Irish border to a “suicide vest”. I find it nothing short of extraordinary that a politician seeking the highest office in the land should deploy such a metaphor, less than 16 months after the Manchester Arena bombing.

And by the way: this has nothing to do with censorship, or freedom of expression. Johnson has the right to say things like this, and the rest of us have the right to deplore him for doing so. The cheeky chap of Have I Got News for You? has morphed into a snarling populist, and we need to approach his ambitions with deadly seriousness.

In which respect, it is a huge strategic error by some of Johnson’s political opponents to attack the instabilities of his private life. Of course it is a misfortune that he and his wife of 25 years are to be divorced: but it is their business, and nobody else’s. The fact that Theresa May’s campaign team compiled a dossier on his sexual conduct in 2016 is pathetic enough. Even worse that an unnamed, senior aide said to the Sunday Times: “It is genuinely offensive and categorically untrue that we have done anything to update this document” (my italics). In other words: it’s perfectly acceptable to prepare a file trashing the personal life of a fellow Conservative.

One of the great improvements in British politics in the past two decades has been the decline of moral puritanism. After the unmitigated disaster of John Major’s “back to basics” initiative, there was a rapid and much-needed change in the terms of trade: sexual orientation, infidelity and marital status are no longer fair game for political attack or legitimate grounds for disqualification from public office. Predatory behaviour is another matter, and, when the #MeToo movement was launched, there was much speculation in Westminster that Johnson would be named and shamed. It is only fair to record that not a shred of evidence has emerged of harassment, assault or otherwise exploitative behaviour.

You might think his adulteries and indiscretions would cost him votes with Tory members in a future leadership contest. They are, it is true, a disproportionately judgmental tribe, socially conservative and in many cases downright reactionary. But more and more of them have encountered divorce in their own families and grasp that the old moral bar set by Tory associations is impossibly high (a similar conclusion was reached by many US evangelical leaders about Trump in the 2016 presidential contest: his personal life might be shambolic, but he was the candidate most likely to deliver what they wanted in America’s culture wars). It is no accident that another darling of the Conservative grass-roots, Jacob Rees-Mogg – a devout Catholic – said that Johnson’s divorce would not make “any difference to the fine regard and admiration people in the Conservative party hold for him. I supported him last time and I don’t regret my decision.”

Johnson’s personal life is the worst reason to oppose his candidacy – and those who hope to stop him need to accept this, and move on. At the Big Tent Ideas festival in Cambridge on Saturday – a mostly but not exclusively Conservative gathering – I heard a refreshingly broad range of mainstream ideas and policy proposals for 21st-century Britain. Indeed, after two years of Brexit toxicity and torpor, the entire event was a much-needed adrenaline shot, and a reminder that politics is about more than implementation periods, no-deal planning and big fat lies on the sides of buses.

Johnson’s campaign pitch is going to be a blend of showbusiness and of populist nativism masquerading as patriotic globalism. Tory moderates – no, don’t laugh – need to raise their game, and do so with absolute urgency. They need to rally around a candidate who is prepared to champion the merits of immigration as well as the management of our borders. Who warmly embraces the pluralism of British society instead of treating social diversity as a pathology to be minimised. Who resists the dream of a fictitious Albion restored, and faces squarely the challenges of today: stagnant pay, increased longevity, automation, the digital revolution, climate change.

I have often praised Ruth Davidson – though she has yet to declare an intention to move from Holyrood to Westminster. I was also much impressed by Penny Mordaunt, the international development secretary, at the Big Tent event this weekend. Her emphasis upon civility, LGBT rights, gender equality, social enterprise, and Britain’s obligations overseas all recommend her as an authentically modern Conservative. I disagree with her on Brexit – but her support for Britain’s departure from the EU may cement her appeal with many Tories who share this position but recoil from Johnson’s unplugged populism.

There is no leadership vacancy and, when there is, there will be other plausible candidates. Johnson’s victory is far from inevitable. But the way to beat him is not to focus pruriently upon his private life. In this context, the relationships that matter are ideological not sexual. It is not blondes we should be worrying about, but Bannon.

• Matthew d’Ancona is a Guardian columnist