Play Video 1:09 Everything that went wrong during Theresa May’s speech – video

We are now well into that most traditional of Tory pastimes, a weekend of plotting aimed at removing a leader. Except this time, the whispering, scheming, texting and WhatsApping comes as a once-iron law of politics crumbles before our eyes.

I’m speaking of the axiom that previously held as firm as a law of physics: that the Conservative party has a will to power so strong, it will subordinate every other consideration, principle or interest to obtain and keep it. It’s this that has made the Tories arguably the most successful political party in the democratic world over the course of nearly two centuries.

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Now, however, the Tory tribe finds its onetime laser-guided focus on power wobbling, because it has seen something it values more. There were many explanations for Theresa May’s anxiety dream made flesh on Wednesday, from lethargic security to a ropey set to a bad cold and dodgy throat, the latter clearly aggravated by anxiety following that P45 prank. But the infection that is truly debilitating her and this government is Brexit.

The fever of Conservative Europhobia runs so hot, it has already burnt through a series of supposedly sacred Tory principles. Tories still boast that they are the Conservative and Unionist party, but when they are told that Brexit imperils the union – by jeopardising both Northern Ireland’s peace and its place in the United Kingdom – they shrug. Tories like to say they cherish the supremacy of parliament; indeed the reassertion of Westminster sovereignty was one of the driving purposes of Brexit. Yet now they whip their MPs to pass an EU withdrawal bill that will strip the Commons of its powers, allowing ministers to channel Henry VIII as they rewrite the law of the land unhindered and unscrutinised. They insist that free trade remains a cardinal principle of modern Conservatism as they rush to leave what is, broadly defined, the largest, freest free-trade area in human history.

Given that they have deemed Brexit to be more important than the union, parliamentary sovereignty and free trade, it shouldn’t come as a shock that Conservatives now regard it as more important than the survival of a Conservative government – more important, perhaps, than the future of the Conservative party itself.

Of course, the arch-leavers would reject the implication that Brexit and their own prospects are in conflict. To their mind, Brexit is destined to be a glorious victory – to rank alongside Agincourt and Trafalgar, promises Jacob Rees-Mogg – for which they will be applauded and rewarded by a grateful nation. But the reality says otherwise.

Start with what is still, despite everything, the most basic determinant of electoral success: economic wellbeing. Brexiteers might want to put their fingers in their ears when they hear the warnings from the Bank of England and the Institute for Fiscal Studies, or indeed the downgraded growth figures from the Office for Budget Responsibility, which will announce next week that it is adjusting its forecasts after persistently overestimating British productivity, a move that will leave an £18bn hole in the public finances.

I watched the audience at a fringe meeting in Manchester this week listen in sullen silence as one former cabinet minister, Dominic Grieve, told them it was impossible to tell if we might be better off in 30 years’ time: all we could be sure of was that Brexit was “a sudden and profound shift”, and therefore a huge economic risk in the short and medium term. He cited the projection that while 70% of all European banking currently goes through Britain, that figure tumbles to 40% after Brexit.

Normally that would chill the blood of a Tory conference aware that at the next general election it will be defending a nonexistent majority against a resurgent Labour party. But these are not normal times. Instead, delegates cheered any speaker who promised the hardest possible Brexit, the more cliff-edge the departure, the better. “We have nothing to fear from no deal,” Rees-Mogg told the rapturous crowd that had filled Manchester Central, even though crashing out on WTO terms would be a devastating shock to the UK economy, and would destroy the electoral hopes of the government that did it. The applause was ecstatic.

For the Europhobes, whose voices sound loudest in the post-referendum Tory party, no Brexit is ever quite hard enough. Not content that Britain is walking out on our nearest neighbours and most important trading partners, they insist we slam the door shut – and give a two-fingered salute on our way out.

All week, MPs and delegates nodded sombrely as they conceded their electoral problem with the young – a term now redefined to include anyone under the age of 47. They know that younger voters are repelled by Brexit, but they persist all the same. The moderniser MP George Freeman shook his head as he told me: “If we’re seen simply as the defenders of soulless austerity and of an ideological and culturally isolationist Brexit, we risk losing the next generation of voters for ever.”

Most know that, at a minimum, they need to offer remedies to student debt and the housing shortage. Both require big, bold moves right now if they are to bear fruit in time for the last possible date with the electorate in 2022. But in between the coughs and disintegrating scenery, May’s speech included only modest tweaks in both areas. Five thousand houses a year will barely make a dent.

But there is no resolve to do more or better. With no Commons majority and no broader sense of purpose, all the available oxygen is taken up by Brexit. Governments have limited bandwidth; there is only so much a finite number of civil servants can do, only so many hours in a Downing Street day. And the Conservatives have decided to devote what energy they have to leaving the European Union.

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So now they’ll spend a weekend conspiring, as if their leader is the chief problem. They will ponder May’s future, debating whether she is now too weakened even to stagger on till March 2019, wondering whether she has the authority to hammer out a collective government position for the negotiations with the EU: incredibly, the cabinet is yet to debate, let alone agree, its stance.

If a leadership contest happens, it will – yet again – be a proxy battle over Europe, the same issue that has poisoned the Tory party, and with it British political life, for decades. It has become the acid in the eyes of British Conservatism, blinding them not only to the national interest but to their own principles – and even their loosening grip on power.

• Jonathan Freedland is a Guardian columnist