Let’s drop the niceties. Cut the pretence. Something is happening to the Tories, obvious even to that vast majority of the public who ignore politics. The Conservative party is becoming the natural party of extremists. It is the new home for hardliners, catastrophists and those wishing to take up permanent residence in la-la land.

Evidence of this mutation is in every day’s headlines, and borne on a never-ending stream of tweets. It is Jacob Rees-Mogg, coolly suggesting that British representatives should run amok and cause chaos throughout the EU. It is openly acknowledged when the chancellor, Philip Hammond, utters a prayer to “flush out the extremists” in the party. And of course it is Mark Francois, doing Mark Francois. Forever auditioning to be lance corporal in a war that ended 20 years before he was born. Ripping up a letter from the CEO of Airbus, saying it was “German bullying”. Barking about “perfidious Albion on speed”, as if they were headlining Glastonbury.

We may have to wait 30 years for the official papers to confirm that Francois was, all along, a baroque comedy character devised by Harry Enfield and then discarded as too ludicrous. Yet there is a deeply serious side to what has happened to the Conservatives. We are now governed by a party whose primary justification is to secure Brexit, and which does no actual governing aside from talking about Brexit. Yet the same party repeatedly rejected the Brexit deal secured by its own leader – and then last week vetoed every alternative to that deal. Theresa May’s response has been to elevate a political crisis with her own party into a constitutional crisis of the state. The prime minister also plans a £120m festival of Brexit, while 4.5 million children live in poverty.

Evidence of this mutation is in every day's headlines. It is in Mark Francois, barking about perfidious Albion on speed

Label this cognitive dissonance, if you wish to be kind. Deem it illogical, if after three years you’re still surprised by such grotesquerie. But whatever you do, don’t call it strong and stable. Don’t kid yourself that this bunch shares much DNA with the party that styled itself the “natural party of government”. No: today’s Britain is shackled to a bunch of just over 300 extremists, whose desire for ideological purity outruns their ability to do anything.

Everyone can see this; it’s just that the political classes are too squeamish to come out and say so. Instead, the Times columnist Matthew Parris refers to hardliners “poisoning” the tribe for whom he was once an MP. That loyal chronicler of the Cameroons, Daniel Finkelstein, now mumbles about his party’s “nervous breakdown”. This is to euphemise away epic drunkenness as getting a little tipsy. These professionally eloquent men never spare the seasoning when alleging appalling motives and outrageous action on Jeremy Corbyn’s side. Yet where Corbyn has restored Labour to its social democratic tradition, it is May’s Tories who have become truly radicalised.

Where today’s Labour party boasts half a million members, there are fewer card-carrying Tories than Mormons in the UK. What’s more, according to research from the London School of Economics, the party was reliant over the past decade on just 15 sources for almost a third of its entire income. The atrophying of Tory democracy has turned the party into a leisure pursuit for the tinhat brigade and a gewgaw for multimillionaires.

In Britain’s decrepit electoral system, there will always be a place for the Conservatives, but what is striking is how they have shrunk into being the party of southern England. And even there, the hold is slipping. This January, a senior government minister told the Sun of a new rule, coined while out canvassing, that “If you knock on a door and they have books on their shelves, you can be pretty sure these days they’re not voting Tory.” The old cliche claims that any man under 30 who is not a liberal has no heart, whereas anyone over 30 who is not a conservative has no brain. No more. Voting Tory is for one’s dotage, as health secretary Matt Hancock remarked this week, voting Tory is now “something people start to do when they get their winter fuel allowance”. And can you imagine anyone seeing in May’s rabble an oasis of sagacity and level-headedness?

Denialists sometimes claim that the Tories’ troubles are confined to a double-breasted fringe of Rees-Mogg and his European Research Group. That turns a blind eye to the most salient facts. First, Brexit will hang over our politics, which is to say dominate the Tory party, for years – and the divorce deal is if anything the less contentious part. Second, May’s would-be replacements know that to stand a chance, they must sound as extreme as possible. Liz Truss was a remainer; now she garbles speeches about the supermarkets’ unpatriotic dearth of British cheese. Fellow former remainer Jeremy Hunt warned only last summer that leaving Europe without a deal would be “a mistake that we would regret for generations”; today he jabs at Brussels like a mini-Francois. In today’s Tory party, there is no moderation or tolerance, no berth for Dominic Grieve or even Nick Boles.

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Finally, the roots of Tory extremism stretch back far further than 2016 and May’s anointment. Many of the key men behind Brexit are back numbers: Michael Howard, who used to give speeches on immigration virtually parroting Ukip language and policy. Iain Duncan Smith, John Redwood, Nigel Lawson. And then there is David Cameron, whose election manifestos contained pledges on immigration numbers so extreme as to be impossible, whose chancellor and friend pushed a perverse economic policy – and who took the Tory MEPs out of the traditional conservative grouping in the European parliament and allied them with a far-right grouping that has included such parties as Germany’s AfD, the Finns party and the Danish People’s party.

A three-time Conservative prime minister, Lord Salisbury, defined his party’s purpose as “hostility to radicalism, incessant, implacable hostility … The fear that radicalism may triumph is the only final cause that the Conservative party can plead for its own existence”. His deadly opponents were the late Victorian liberals, yet his party has today been hijacked by the radicals. It no longer has space for the Keynesianism of Rab Butler or the liberalism of Francis Pym. It is the home of the radical right. That defines the Brexit project; it is also the dying embers of Thatcherism.

And so a long and honourable tradition in British politics dies in front of us, in plain sight. Look at the men and women vying to replace May and they look little different from the Tories you used to know. Their suits are as traditionally roomy, the haircuts just as artless and the complexion as reliably white. But the party they wish to head is now a haven for conspiracy theorists, cranks and career cowards.

• Aditya Chakrabortty is a Guardian columnist