Half a league, half a league,

Half a league onward,

All in the valley of Death

Rode the six hundred.

“Forward, the Light Brigade!

Charge for the guns!” he said.

Into the valley of Death

Rode the six hundred.

As Theresa May rides towards an all but certain defeat in parliament this week, some of her more historically minded colleagues are likening the prime minister to the doomed Charge of the Light Brigade. That’s a little unfair. The Crimean war commanders of that suicidal assault on Russian guns didn’t know they had ordered a disaster. By contrast, Mrs May is fully conscious that she is galloping into the valley of parliamentary death. She has been given ample warnings that the cannon of the hard Brexiters are to the right of her, the cannon of the unreconciled Remainers are to the left, and the cannon of the opposition parties are in front of her. She won’t be able to say she wasn’t told. She will not be able to make a fall guy of the chief whip, Julian Smith, and blame him for messing up the maths. He’s done the numbers and told the prime minister that the chances of success are bleak.

This is a highly unusual event in the annals of politics: a prime minister is heading knowingly towards defeat. The only previous occasion in recent history that I recall this happening was when Tony Blair was in the no-one-likes-me-and-I-don’t-care phase of a long premiership. He pressed ahead with a vote on anti-terrorism legislation, which his whips had told him he couldn’t win. He duly lost, a defeat that was a contributory factor in his departure from Downing Street.

So, on the face of it, Mrs May is being wildly reckless when she insists that she will press ahead with the parliamentary vote on her Brexit deal this Tuesday even though no one thinks she can possibly win it. She faces defeat partly because her deal has very little appeal to anyone. The report of the all-party select committee on Brexit, published today, joins the chorus denouncing the withdrawal agreement as the worst of all worlds. Even those Tory MPs who are offering support to the prime minister are doing so with fingers clamped on noses, backing her only because they like the alternatives even less or fear constitutional chaos. Defeat beckons for Mrs May also because Number 10’s strategy for manipulating the argument in its favour has not worked. The prime minister and the cabinet colleagues prepared to speak for her have framed the choice as “this deal, no deal or no Brexit”. Her hope was that Brexiters would be scared into line for fear of losing their prize and Remainers would be frightened into supporting her deal in dread of Britain going off the edge of the cliff without a parachute. This tactic has worked on some MPs, but has had the reverse effect on others. The Brexit ultras will vote against her because they want a no deal or believe, on no evidence whatsoever, that rejecting this agreement will magically stimulate improved terms from the EU. The government’s tactics have simultaneously emboldened those who seek to soften or reverse Brexit. They will vote against the deal in the hope of getting a different kind of departure from the EU or paving the way to another referendum.

Having come to their own conclusions that the government is staring at defeat, some in the cabinet have been trying to persuade Mrs May to retreat by postponing the vote. So has Sir Graham Brady, the important chairman of the 1922 Committee of Tory backbenchers and a consistent friend to Mrs May during her premiership. I thought it significant that Sir Graham, normally the acme of discretion, went public with his suggestion that she ought to delay the vote.

As I write, she is displaying no signs of heeding this advice, even when it is tendered by allies. Many will say that this is characteristic of Mrs May, a woman who often acts as if there is no greater virtue than to be stubborn. Yet in this case she may well be correct to calculate that there is nothing to be gained by trying to evade the moment of truth in parliament. There is no reason to suppose that delay will suddenly induce a fresh offer from the EU that will be so much more palatable to the Conservative party that it will significantly reduce the cohorts of backbench rebels. Some have suggested that the vote be put off to later in the month in the hope that Mrs May might extract some last-gasp concessions at the European council meeting this Thursday. But members of the cabinet worry, with good reason, that this could set her up for more humiliation because European leaders will give her a dusty answer and she will return from Brussels with nothing.

There are no good grounds for thinking that a bit more time would give Mr Smith an opportunity to substantially change the parliamentary maths. The usual levers of the government’s enforcers aren’t working in this context. The issue is too momentous; their power is too feeble. You can’t threaten a rebel MP with career-terminating consequences if he or she has already resigned from the government payroll to vote against the prime minister. There are far too many Conservative MPs against the deal and they are far too entrenched in their views for an extra week or so to make a meaningful difference.

In normal circumstances, we would expect the resignation of the prime minister to follow a defeat of such magnitude

Pressing on with the vote comes with great risks for Mrs May. If parliament rejects her deal, she will have been repulsed on the defining task of her premiership. That is big. In normal circumstances, we would expect the resignation of the prime minister to follow a defeat of such magnitude. But these circumstances aren’t normal. Brexit has suspended, distorted or inverted the laws of political physics. It is life, Jim, but not as we are accustomed to it. Last week the government was held in contempt of parliament for its initial refusal to publish the attorney general’s legal advice on the withdrawal agreement. That was another of the baleful precedent-busters that have been unleashed by Brexit. Yet no one resigned – and there was no expectation that anyone would do so. All our usual assumptions about how politics is supposed to function have been torn apart by the Brexit maelstrom.

In normal circumstances, a leader would be expected to quit because defeat on the central plank of the government’s programme would be held to have shredded prime ministerial credibility. But this rule doesn’t really apply in Mrs May’s case because her authority has been shattered multiple times already. You can’t lose what you’ve already lost.

She might face the challenge to her leadership of the Tory party that the blowhard Brexiters have kept threatening without ever delivering. Or it is possible that Mrs May might feel impelled to leave of her own volition if she is defeated by a crushing margin. It is conceivable that she could declare that she has tried her best, express regret that parliament can’t agree, and announce that she is leaving the stage to let someone else see if they can do any better. That’s not impossible, but quitting in that fashion would be out of character. Walking away from messes of their own creation is what juvenile males like David Cameron and Boris Johnson do. Whatever transpires on Tuesday, it is not an absolute given that she will have to quit. Nor is there evidence that this is what voters will expect of her. The pollsters are reporting a paradoxical mood among the public: the majority don’t like her deal, but only a minority think she ought to resign if it goes down in parliament.

Beyond the relatively trivial question of what happens to the prime minister, there is a compelling argument for getting on with this vote in the national interest. You may have noticed that time is desperately short. There is a little over three months left before Britain is scheduled to leave the EU. If Mrs May’s deal can’t be got through parliament, that needs to be established as rapidly as possible so that MPs can start trying to navigate towards a non-catastrophic resolution of this nightmare. That could mean attempting to find agreement on a different form of Brexit or throwing the question back to the people with another referendum. Defeat for the prime minister’s deal will open up possible routes of escape from this hideous trap.

We have reached a point in the Brexit derangement where charging into the mouth of hell is perhaps not crazy at all, but the sanest thing that Mrs May can do for herself and her country.

• Andrew Rawnsley is an Observer columnist