Strange to say this of a country with a Tory government, but Britain lacks a conservative party. There are instead two revolutionary parties of the right, firmly set on a path that could be described as nationalist or populist. But it is avowedly not conservative. And now they are locked in a struggle that is dragging the older, larger party ever further from the values that are supposed to define it.

That struggle would have been even more vivid if Nigel Farage’s Brexit party had won the Peterborough byelection. It would have filled Tory MPs with terror that Farage was coming to take their seats away, directly. Now they are consumed with a fear that amounts to the same thing: that the Brexit party will split the right-of-centre vote in sufficient numbers that even if Labour sees its own support collapse in dozens of marginals, Jeremy Corbyn’s party could still come through the middle and win – just as it did in Peterborough. Either way, the Conservative party is now gripped by fear of Farage. It is galloping towards the conclusion that it must offer what he offers, or else be swept aside. As Boris Johnson, the bookies’ favourite to be the next prime minister, tweeted in the dead of night after the Peterborough defeat: “Conservatives must deliver Brexit by 31st October or we risk Brexit party votes delivering Corbyn to No 10.”

The Conservative party is galloping toward the conclusion that it must offer what Farage offers, or else be swept aside

In that stampede, the Tory party is moving ever further away from what should be its defining set of beliefs. As any of the thousands of politics A-level students who took their exams this week could tell you, conservatism is meant to be the ideology wary of ideology, a creed whose core beliefs include pragmatism, gradualism and a wary scepticism of grand schemes and visions of perfection.

The conservatism of Edmund Burke and Michael Oakeshott prefers the slow, organic and incremental to the radical and sweeping. It cherishes institutions constructed carefully over decades, and steers clear of upheavals that promise to tear down the old order and start anew. It prefers the mature garden to the clean slate. In Oakeshott’s words: “To be conservative, then, is to prefer the familiar to the unknown, to prefer the tried to the untried, fact to mystery, the actual to the possible, the limited to the unbounded, the near to the distant, the sufficient to the superabundant, the convenient to the perfect, present laughter to utopian bliss.”

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The Brexit party is right wing, to be sure, a populist nationalist movement railing against a hated elite. But it is anything but conservative. Its philosophy is slash and burn. It looks at the dense tangle of roots and connections grown thick over nearly half a century of British membership of the European Union, and wants to rip the entire thing out of the ground. It pays no heed to precedent or evidence or basic economics, preferring instead to get high on abstract nouns like “democracy” and “freedom”. Small wonder that the former Revolutionary Communist party luminary Claire Fox is now a Brexit party MEP: as a revolutionary, she is among kindred spirits.

And now the Conservative party is following Farage’s lead. This week a former cabinet minister and would-be prime minister, Dominic Raab, threatened to suspend parliament if he won his party’s leadership, so that MPs could no longer stand in the way of his preferred no-deal Brexit. That would be as direct an assault on parliamentary sovereignty as one could imagine. Never mind that the supremacy of parliament was for centuries the lodestar of British conservatism. Never mind that restoring primacy to the Commons was supposedly the animating purpose of the Brexit cause itself. All of that must be torn down for the revolution that is Brexit, which as a matter of theology cannot come even a moment after the hour strikes for Halloween.

Of course Raab was denounced by several in his party – those who will be on the losing side in the current contest – and slapped down by the Speaker, but Raab’s is only a more macho version of the pledge to leave come what may made by Johnson, Andrea Leadsom and all those courting the votes of the revolutionaries in the European Research Group, led by the monocled mutineer himself, Jacob Rees-Mogg. They are all in the grip of a dogmatic certainty about Brexit that would have Burke and Oakeshott reaching for the smelling salts.

The cause of this Conservative rush to revolution is not the Brexit party, even if that is the current spur. It is Brexit itself. Everything about that project forces conservatives to abandon what should be the core tenets of their faith. The Tories are meant to be the party of the union, yet their determination to leave the EU has them jeopardising the United Kingdom, whether by driving Scotland towards independence or Northern Ireland towards unity with its southern neighbour.

They are meant to be the respecter of law and treaty, yet not only were they cavalier in their dismissal of the withdrawal agreement reached by their own government with the 27 remaining EU states, they are casual in their disregard for the Good Friday agreement, a text that any self-respecting conservative should revere for its role in preserving life and limb.

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Today’s Brexiters claim to believe in the conservative principle of free trade, yet they are gagging to leave what is, broadly defined, the largest, freest free-trade area in human history. The result will be to leave Britain as a lone minnow when it faces the shark of Donald Trump in talks for a post-Brexit US-UK trade deal, a shark who this week confirmed that the NHS will be on the menu – even though the health service is just the kind of tested, deep-rooted British institution a true conservative would want to protect.

What underpins this recklessness is an ignorance, or ignoring, of history – even though humility before the past should be a defining trait of the conservative. That was especially clear this week, as world leaders gathered to mark the 75th anniversary of the D-day landings. Those ceremonies were a reminder of what remains the founding and ultimate argument for the European Union: that if the nations of Europe don’t co-operate with each other they will fight each other until tens of millions are dead. Heed the words of Eric Chardin, the 94-year-old D-Day veteran who movingly told the BBC that he didn’t want Britain to leave the EU, because he wanted the peace that followed the second world war to endure: “We’ve gone to so much trouble to collect the European big nations together, to break it all up now would be a crying shame.”

The Conservatives are readying themselves for election battle against Corbyn, but what could have been one of their most potent weapons now lies blunted in their hand. They cannot warn that Corbyn is a dangerous revolutionary, any more than they could ever again repeat their promise of strong and stable government. For they are dangerous revolutionaries now too. Just as their own record on Islamophobia prevents them launching a credible attack on Labour antisemitism – and note the depressing fact that the new MP for Peterborough gave an online nod to talk of “Zionist slave masters”, apologising once it came to light – so they cannot accuse Labour of wanting to tear down the existing order in a rampage fuelled by ideological zeal. They cannot make that accusation because they would be describing themselves.

This is what happens when you allow Nigel Farage to dictate the terms of political trade. A party that once presented itself as the guardian of cautious common sense is now consumed by fervour, ready to burn everything down, and all for the promise of distant, utopian bliss.

• Jonathan Freedland is a Guardian columnist