She ought to win the vote of confidence, but that doesn’t mean she will. Most Conservative MPs, it has to be remembered, voted Remain in the referendum and want to see a soft Brexit – one that leaves us in a close economic relationship with the EU.

They won’t want to hand the decision over who should be prime minister to the party members, who are more likely to choose a hard Brexiter. That is why Boris Johnson and Dominic Raab – both of whom are prepared to leave the EU without a deal – are the favourites at the bookmakers to be the next Tory leader.

So Conservative MPs ought to rally behind Theresa May, however much they despair of her ability to turn things round. The question for them to decide by 8pm tonight is whether the Brexit situation is so hopeless that they might as well throw the dice and see what happens.

That would seem to be a step into the unknown. If they remove her as leader, they cannot even be sure that her successor would be prime minister.

One of the campaign lines against Johnson during today will be to point out that Dominic Grieve and Anna Soubry have threatened to quit the party if he becomes leader. That means the Conservatives would be further away from a majority, and in any case the DUP would have to negotiate with the new leader.

Most Conservative MPs must also fear that changing their leader would not change the Brexit numbers in the House of Commons, nor would it alter the EU’s negotiating position. The prime minister says that any withdrawal agreement must include a backstop – the guarantee of an open border in Ireland – which is what Tory opponents of her deal don’t like. It would be quite a gamble to assume that a different prime minister would be able to change the backstop in ways May was unable to do after a year of hard negotiation – simply by making a threat of walking away, a threat that lacks basic credibility.

The argument being made by supporters of Johnson and Raab is that a new leader might change the chemistry in some mysterious and unknowable way. Johnson in particular, with his admiration for Winston Churchill, the discredited voice in the wilderness who turned out to be exactly what the nation needed in its hour of crisis, seems to believe that he can untie the Gordian knot of Brexit through sheer force of personality.

If May loses the vote tonight, the Brexit deal she has negotiated would be hard to revive. Regardless of the choice facing Tory MPs and then party members in the next stage of a leadership election, parliament would face a choice between a no-deal Brexit and postponing Brexit in order to hold a referendum. That couldn’t happen under Johnson or Raab, which raises the prospect of a split in the Tory party in parliament. Faced with a no-deal Brexit, some Tory MPs may look to a Labour government or a “government of national unity” to avert what they would regard as a national disaster.