There’s a theory at Westminster that Theresa May is a much cleverer politician than she looks. The world sees a second-rate prime minister trapped in a context that she lacks the strength and agility to escape. That is the cunning disguise adopted by a maestro of manipulation with a remarkable talent for survival. Machiavelli has a sister and she lives at Number 10.

She is ruthless at dispatching wounded ministers when it suits her and as ruthless at retaining damaged ministers (Amber Rudd being the topical example) when they can serve the prime minister as human shields. She threw away the Tories’ majority at an election she didn’t have to call and yet somehow endures in Downing Street by convincing her party that she is the only one among them who can keep this rackety government on a rocky road strewn with danger.

Consider, most of all, Brexit. It is more than a year since Britain formally told the EU that it was leaving and there are now less than 12 months remaining before departure is due to become historical fact. Many have been the predictions that Mrs May could not possibly keep her brittle government and riven party together through this test. Yet those forecasts have so far been confounded. Her cabinet have fought like rabid ferrets, but not one of them has actually flounced out over Brexit.

A guileful prime minister – so goes the theory I have heard from some Tory MPs – has slowly but surely stitched up the hard Brexiters. She made prisoners of David Davis, Liam Fox and Boris Johnson by giving them nominal charge of the negotiations while ensuring that she has called all the essential shots. She took their “red lines” and pretended to agree with them and then let the power of time and the brute facts of the parliamentary arithmetic and economic life do the work of softening Brexit. Those “red lines” have often times been smudged and in some cases erased altogether.

The negotiating process with the EU has been punctuated by regular British retreats from the hard-line positions initially advocated by the Brexit fundamentalists. This has involved degrees of humiliation for Mrs May, but nothing that her armoured hide cannot absorb and nothing compared with what has been inflicted on the Brexiters. From fisheries to regulatory alignments, they’ve been forced to eat plates groaning with their previous promises.

Where once many of their number declared that they would not countenance paying a red cent to the EU, Britain will be handing over a hefty divorce settlement. Where once they ringed the departure date as “liberation day”, they have conceded to a lengthy standstill transition, which means that EU law will prevail in Britain at least until the end of 2020. “Take back control”, the chest-beating slogan of their referendum campaign, has become feeble bleats about “implementation periods” and “managed divergence”. The longer the negotiations have gone on, the more and more evident has it become that there is no bargain with the EU that Britain can strike that is as advantageous as the deal it currently enjoys as a member, not at all the conclusion that the Brexiters wanted to become apparent. As the concessions to reality have piled up, they have huffed and puffed, but never made good on threats to blow the house down. Nigel Farage has been left in a lonely corner of the union jack bar to weep into his beer between moans of betrayal.

And now they’ve had enough. The hard Brexiters declare there will be no more surrenders. Like the crazed newscaster in Network, they rage: “I’m mad as hell – and I’m not gonna take it anymore.” There is a red line, they pronounce, which they will not allow the government to cross. If it does, so they threaten, they will tear down the Tory temple.

Facebook Twitter Pinterest Like the crazed newscaster in Network, they rage: “I’m mad as hell – and I’m not gonna take it anymore.” Photograph: Allstar/MGM

That red line is British membership of a customs union after Brexit. Boris Johnson selected that as the ditch he would die for when he said that remaining within a customs union “would leave the UK a colony of the EU”. In a speech a couple of months ago, Liam Fox termed it “a complete sell-out of Britain’s interests”. It is hard to see how either could stay in a cabinet that negotiated the very thing they have so passionately denounced.

Their problem is that they don’t speak for parliament. A recent defeat for the government in the House of Lords, where a substantial majority of peers think Britain should protect its trade with the EU by remaining within a customs union, was an early warning of how much peril this issue poses for Mrs May. As I have remarked before, there is a natural majority in the Commons for remaining within some form of customs union, now that Labour has adopted that course. The great majority of trade unions and most businesses support the idea. The government’s own analysis, which was helpfully leaked, suggests that losses to commerce with our closest neighbours can never be compensated for by the putative gains to be had from negotiating unilateral trade deals with third countries from outside the customs union. Business would rather have the bird in the hand than hypothetical fledglings in theoretical bushes to be raised at some unspecified date in the distant future.

Polling suggests that it would be fine with most voters as well. “The will of the people”, that much-loved trope of the hard Brexiters, would not be defied by parliament instructing the government to keep Britain in a customs union. Then there is the wicked issue of the Irish border, a conundrum that induces a migraine in everyone who examines it. Remaining within a customs union doesn’t entirely solve that problem, but it goes a long way to making it fixable.

Quite a lot of the cabinet would not be terribly upset if parliament told the government to stay in a customs union. The chancellor, Philip Hammond, and the business secretary, Greg Clark, would be able to contain their distress if the government was defeated on the issue. Likewise the de facto deputy prime minister, David Lidington.

Mrs May’s avowed intention is to leave the customs union and she has said so often and in such unequivocal language that it doesn’t leave her any apparent wriggle room. As recently as last week’s prime minister’s questions, she stuck with the hard Brexiters’ interpretation of the referendum result when she declared: “The British people voted to leave the European Union. In voting to leave the European Union, they voted to leave the single market and the customs union.”

On the face of it, that would make it extremely difficult for her to negotiate a customs union. So why do hard Brexiters not trust her and why do some of her cabinet speculate that she could embrace the idea if parliament forced her to? Because Mrs May has a reputation for standing on her head while swearing that nothing has changed. You will recall that she said that she was not going to have an early election because it would be contrary to the national interest. This was a position that she maintained relentlessly, all the way up to the point when she suddenly declared that it was in the national interest to have an early election.

The hard Brexiters are clearly fearful that there will be a similar volte-face over the EU negotiations and that Britain will end up in a customs union, or something very similar by a slightly different name. So they are trying to put a gun to her head by predicting that it will trigger cabinet resignations, which I am pretty sure it would, and a full-scale Tory civil war, which would blow away Mrs May’s premiership. They menace her with a confidence vote in her leadership. That wouldn’t change the parliamentary arithmetic, which is the fundamental weakness in their position. Nor would a confidence vote necessarily remove her as prime minister, but it is not a prospect she wants to face, which is why they make the threat.

The prime minister has not found an answer to this crunch other than to try to delay it. The government has stalled a chunk of the withdrawal legislation to try to swerve the moment of truth, but it won’t be able to duck a binding vote forever. The great rupture of the Tory party over Brexit cannot be avoided indefinitely. What might look to some like cunning cleverness is, on closer inspection, really an exercise in trying to postpone the inevitable while desperately hoping that something will turn up. Not so much Machiavelli as Mr Micawber.

• Andrew Rawnsley is an Observer columnist