There was a time when Conservatives used to split over this or that aspect of an EU treaty; or the practice of monetarism; or the composition of the House of Lords; or the consequences of welfare policy. Now they argue over whether it is acceptable to sneer at Muslim women in religious dress. O tempora, o mores, as Jacob Rees-Mogg might say.

There is depressing bathos in the fact that two cheap gags in Boris Johnson’s Daily Telegraph column about the burqa have caused such a rift. But they have.

The former foreign secretary refuses to apologise for writing that a Muslim woman wearing the veil resembles “a bank robber” and that it is “absolutely ridiculous that people should choose to go around looking like letter boxes”. He is now under investigation by his own party, which has received dozens of complaints.

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According to Rees-Mogg, this inquiry is nothing more than a “show trial”, animated by envy of Johnson’s “many successes, popularity with voters and charisma”. Four cabinet ministers to whom the Sunday Times spoke are reportedly unhappy about the Tory leadership’s “cack-handed” handling of the furore. Andrew Bridgen, Iain Duncan Smith and other Conservative MPs have raced to support Johnson.

On the other side Andrew Cooper, a Conservative peer and former No 10 director of strategy, tweets that “the rottenness of Boris Johnson goes deeper even than his casual racism & his equally casual courting of fascism”. Ruth Davidson, the Scottish Tory leader, has demanded an apology for his “gratuitously offensive” remarks. Dominic Grieve, the former attorney general, has said he will leave the Conservative party if Johnson becomes its leader.

Let’s first establish what this row is not about. For a start, it is emphatically not about freedom of speech. In absolutely no way has Johnson’s liberty to speak out been forcibly curtailed or legally constrained. On Thursday the Met police commissioner disclosed her “preliminary view” that “what Mr Johnson said would not reach the bar for a criminal offence”. There will be no campaign to free the Bullingdon One (though you can bet that the populist right would love nothing more).

Nor is this an example of our old friend, “political correctness gone mad”. The debate about the place of the niqab and burqa in a liberal secular society long preceded Johnson’s clodhopping contribution, and will carry on long after he heads off greedily to the next opportunity for self-advancement.

Nor is the row about the limits of comedy. It pains me to criticise Rowan Atkinson, whose portrayal of the mime, “Alternative Carpark”, is one of the greatest achievements of British humour. But his defence of “the freedom to make jokes about religion” in the Times last week was unnecessary. That freedom has not been imperilled by this particular case. What Atkinson’s letter does reveal is a dangerous confusion between the functions of the comedian and of the politician, between entertainment and statecraft. Politicians have always told jokes, and a fair few have become them. But – as Donald Trump, the reality TV star turned President has shown – showbusiness has now fully colonised the world of politics.

In Johnson’s case, what we are witnessing is a man who until very recently held one of the great offices of state claiming jester’s privilege. When he writes or speaks, he does so as an MP and privy counsellor well known for his ambition to become prime minister in the near future. To borrow the language of King Lear, he plays the Fool but wants to be Nuncle. This may be clever, but it is also intolerable.

It is almost a working definition of politics: you can say what you like, within the law, but you must then face the consequences. A standup comedian will point out that he is just making jokes, or being ironic or testing taboos. A politician does not enjoy such licence – indeed how could he or she? A political party is not a free-for-all or a convenient platform for uncurtailed rhetoric, but an organisation for the disciplined achievement of power and implementation of principled policy. It is this basic structural reality that Johnson has never respected.

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The problem is that the spirit of the age is on his side. In an era of digital networks, the old systems and structures are crumbling fast. When Enoch Powell made his notorious “rivers of blood” speech, the then Conservative party leader, Edward Heath, was able to despatch him to the fringes of political life with relative ease. But the new contours of the web mean that there are no fringes: only voices, and those eager to heed them. This Johnson understands all too well, urged on by Steve Bannon, Trump’s former chief strategist – who praises him in the Sunday Times for “giving the people what they want – authenticity”.

What a weasel word that has become. It started life as a near-synonym for “sincerity” or “honesty”, the opposite of spin. Now it has become code for “giving the voters permission to feel all right about their most irrational hatreds and least honourable emotions”. In this case, Johnson’s “authenticity” resides in his signal that is acceptable to use demeaning, dehumanising language about Muslim women in religious clothing.

I think – no, I insist – that this is nothing short of deplorable, and that this confrontation poses greater long-term dangers than Powell’s speech in 1968. It is a founding principle of any pluralist society that in our permanent negotiation with one another we strive to be decent and dignified.

But that principle will not defend itself. We have reached a fork in the road where it is under sustained attack from nativists, opportunists and bigots in suits. There are only two paths available. Johnson has chosen. So must we.

• Matthew d’Ancona is a Guardian columnist.