Conservative no confidence vote is Sliding Doors moment

If the Prime Minister wins the support of a majority of her MPs, her efforts to get a Brexit deal through Parliament go on as before. True, if the margin of victory were small, she might step down – but she might well not.

At the very least, her premiership has been a display of resilience: that essential ability in politics to keep getting out of bed in the morning and getting on with it when the world – and close colleagues – are hurling insults.

She managed a joke today in Prime Minister’s Questions about what one MP suggested to her was “just a normal day in the office”, offering (to laughter) that she would today have “meetings with colleagues – maybe many meetings”.

Even if she survived the attempt by some Conservative MPs to get her out, getting Parliament’s support for a deal would still remain a formidable task. She has promised to put a deal with the EU to a ‘meaningful vote’ in Parliament before 21 January. Many MPs are determined to attach amendments if so – at least one of which might call for a second referendum.

There was not a majority on Tuesday in support of her deal when the Government pulled the planned vote. What could change to persuade enough MPs to support a deal? Brussels and European leaders have given no sign (and that is an understatement) that they are prepared to make significant concessions on the backstop to the Irish border, still the most objectionable part of the deal for many MPs. It might only be fear of 'no deal' that would cause a majority to form – the clock ticking down to 29 March.

But the growing talk in Westminster about asking the 27 European countries to extend the Article 50 deadline might in turn undermine the willingness of MPs to hold their noses and vote for the deal. Yet that is a slippery calculation.

A bid for more time would bring its own uncertainties and perhaps make little difference. The request would need the backing of all 27 other European countries. They would not want to give much of an extension, as they do not want the UK to field a slate of candidates for the European Parliament elections in May.

Jumping across into the parallel world, if Theresa May lost the party’s confidence vote, or chose to step down, more possibilities open. Many of the candidates to replace her will position themselves as Brexiteers, or at least will be willing to forge ahead with the process of leaving. The question is whether one who would comfortably contemplate a 'no deal' Brexit won through. That could lead the Government to pursue a much harder line in demanding changes from Europe, with the clear willingness to embrace 'no deal' if those were rejected – as they very likely would be.

The sheer length of the full leadership election would bring a first problem. It could well take a month and a half – beyond the 'deadline' of 21 January. That is the date when Parliament (following a key vote last week) has a chance to shape what the Government next does – if there is no deal with the EU by then.

This is where the footwork gets tricky. If the leadership election were still going on by 21 January, and Theresa May’s deal with the EU still counted as a “deal in principle”, then MPs would not have an opportunity to intervene. She would not have brought the Commons debate that she had promised. MPs would not have any chance to amend a motion.

If the leadership election had concluded, however, and the new leader had run on a platform of renegotiating the deal, then in theory MPs might have a chance to intervene through a debate and amendments to a motion. Even so, any amendments would not be binding on the Government although they would have political weight.

Would a new Prime Minister ever be able to get support for a 'no deal' exit in Parliament? On the face of it, he or she would seem to face a huge hurdle. One of the few certainties of the past months has seemed to be that Parliament agrees on only one thing: a majority of MPs do not want 'no deal'.

In theory, MPs now have some power to shape what the Government would do next if there were no deal by 21 January. But that would depend on enough of them agreeing a course. There remains a possibility that if the new Prime Minister wanted to let the time until 29 March expire, that is what would happen. There might be little Parliament could do about it.