If there is a better illustration of the need to be careful what you wish for than this week’s Conservative conference, I haven’t seen it. The Birmingham symphony hall is packed with people who campaigned for Brexit, are getting Brexit, and look most unhappy about it. In less than six months, the UK’s membership of the EU ends, yet the sect that should be quivering in anticipation of Eurosceptic rapture is glum in Brum.

It would be wrong to say the Tories are panicking. They are at the brittle stage immediately before panic, just about holding it together. Ministers wear the taut stares of people who might burst into maniacal laughter or hysterical tears. The terms of EU withdrawal are due to be finalised within weeks and talks are at an impasse. There is not a deal available that can satisfy the whole of the Conservative party, avoid incinerating relations with our continental neighbours, and avert economic chaos.

A popular game on the margins of the conference is guessing the number of irreconcilable Tory wreckers – the MPs who would launch the UK off the Brexit cliff without a legal safety net. The boasters say it is around 80. Their level-headed colleagues cut that estimate by up to a half. Some predict it could be whittled down to 20 once the grave implications of sabotaging a signed agreement are met at close quarters. Feasible consequences of Theresa May’s deal failing include parliament mandating a softer Brexit, a referendum leading to no Brexit at all, or some sequence of tumbling dominoes that propels Jeremy Corbyn into Downing Street as prime minister. Mulling that menu, some ultra-Eurosceptics will fall into line.

Even then, May cannot be sure of a majority, so it is wise to pencil in a political and constitutional crisis before Christmas. Meanwhile, the gravity of the moment works to the prime minister’s advantage. The ticking clock favours an incumbent, even when many Tories long for different leadership. The same backbench rabble-rousers who claim to have a whole battalion poised to overwhelm May’s Chequers deal admit there is not the equivalent will to initiate a vote of no confidence in the prime minister.

It takes 48 letters from MPs to the chairman of the backbench 1922 committee to trigger a ballot that she would probably win. It is a measure of the hardliners’ cowardice and cynicism that they do not want to replace their leader, but bully her into delivering Brexit on their terms. Then they can dispose of her (and blame her when nothing turns out as promised).

This is bad news for Boris Johnson, as the most prominent alternative leader. His most recognisable trait is unseriousness, which is poorly matched to such serious times. Johnson has been loitering with intent to be leader for a bit too long. He is still a favourite of grassroots Tory members, able to pull an admiring crowd here in Birmingham, and it is rash ever to write him off. But he has amassed enemies among the MPs whose backing he would need to advance in a leadership race. There is a sense now of Conservatives sniffing at the product labelled “Boris”, as if investigating a carton of milk that has been open for a few days and might be on the turn.

Johnson is smart enough to know this, which is why there is a certain desperation to his attention-grabbing antics. He is aware that Tory noses are twitching towards fresher candidacies. Among cabinet ministers, Sajid Javid, Jeremy Hunt and Dominic Raab all gave speeches that were reviewed as early-stage auditions for the top job. The speculation also takes in Esther McVey, Liam Fox, Michael Gove … so many in fact that it is more efficient to name high-profile Conservatives who aren’t discussed as part of the post-May contenders’ parade.

This proliferation of wannabe leaders does not reflect fecund Conservative thinking but the opposite. It looks like wild flailing around in the hope of alighting on a saviour. It presages the kind of ragged succession battle when medieval monarchs died heirless. The remnants of David Cameron’s modernising, liberal Toryism expired with the defeat of the remain campaign. Brexit then trampled all over May’s promises to lead a government of social reform, to douse “burning injustice”.

Meanwhile, the party faces a trajectory of demographic decline. Strong indicators of “definitely not voting Conservative” are youth, university education, non-white background and living in rented accommodation. Many Tories know they need something to say to those growing audiences, but their intellectual bandwidth is hogged by the complexity of disentangling the UK from EU membership. And that is a policy that promises nothing to millions of people the Tories have to woo.

The puritanical hunt for a clean break from Brussels, which turns inevitably to the hounding of pro-European “traitors”, aggravates suspicion that the party is more interested in flattering the resentments of its existing voters than befriending new ones.

Conservatives crave better responses to Corbyn’s Labour party than the stale reds-under-the-bed denunciations rehearsed from the stage in Birmingham. But they struggle to imagine what such a message sounds like. They cannot see a clear political horizon because the view is blocked by Brexit.

The Tories have hitched their integrity and their reputation for competence to a project that will deliver them to ridicule before it offers renewal. So they stick with May, not because they like her or trust her, but because someone has to finish the job she has started, and it might as well be her. None of the alternative candidates is fool enough to take on that errand now. So May’s torment continues, her thankless, pointless mission unchanged: to make Eurosceptic Tories unhappy by giving them the thing they always wanted.

• Rafael Behr is a Guardian columnist