Will Theresa May have to quit if she loses the Brexit vote?

Our chief political commentator assesses the prime minister’s chances of survival if she is defeated on Tuesday

The short answer is no, but the longer answer is that no one knows what might happen. Philip Cowley, professor of politics at Queen Mary, University of London, suggested on Saturday that Theresa May might announce her resignation after the “largest genuine defeat on a substantive vote for at least 100 years”, with a new leader being elected and taking over as prime minister in due course.

But everything we know about May suggests it is not in her character to give up unless she is forced to, and there is nothing in our uncodified constitution that means she would have to go. So let us assume that she will try to carry on.

Then the question is whether her party will move against her. There is a volatile psychology to politics at times like these, and so the scale of her defeat, and what she says and does immediately after the vote is lost, could make all the difference.

Conservative MPs could finally launch a challenge to her leadership. The fabled 48 letters might at last materialise in the office of Sir Graham Brady, the chair of the 1922 Committee of Tory backbenchers.

These are the letters from Tory MPs asking for a vote of no confidence in the leader of the party. Under the rules brought in by William Hague in 1998, if 15 per cent of Tory MPs ask for a vote, the 1922 Committee has to hold it, which the leader could win or lose by a simple majority. That means 158 of the 315 Tory MPs have to vote against May to force a contest to elect a new leader, in which she would not be allowed to stand.

So far, the prime minister’s vociferous critics have made fools of themselves by failing to round up even the 48 needed to start the process, let alone the 158 needed to oust her.

But, by triggering the vote at such a moment of crisis and confusion, it may be that the group psychology of the parliamentary Tory party becomes unstable and enough MPs decide for different reasons that they need a new leader to make progress.

It would take a few weeks to elect a new leader. First the MPs would hold repeated ballots to reduce the field to two candidates, then those two names would go to a ballot of all party members.

That means the new prime minister would probably have to ask the EU to postpone Brexit while they tried to negotiate their way through the unchanged parliamentary arithmetic.

As for who the new leader would be, that really is anyone’s guess. It’s often assumed that Boris Johnson is unpopular with MPs, which means he would struggle to make it to the last stage, but that whoever is the more Eurosceptic of the last two would win among party members. For what it is worth, the betting markets have Dominic Raab and Sajid Javid as joint favourites, followed by Michael Gove, Boris Johnson and Jeremy Hunt.