A Brexit vote that changes nothing and possibly everything

Theresa May will lose the vote tonight on her Brexit plan, widely seen as the most important vote in parliament since the early years of the Second World War, and yet nothing of importance may change - or at least not immediately, at least.

How can that possibly be - especially since she could well lose by a record-busting and humiliating margin of more than 100 votes?

It is because she is very unlikely to acknowledge that her deal is dead, and will instead announce shortly after the defeat that she will have another go at negotiating with EU leaders to amend it so as to make it acceptable to MPs.

To be clear this is surmise, though based on conversations with officials and ministers.

Theresa May and Jeremy Corbyn in the House of Commons. Credit: PA

The point is on this most momentous of decisions for the UK, the prime minister has not even yet shared her plans for a Plan B with ANY colleagues. And is not expected to do so, even at today's cabinet meeting, for fear that her not-so-discreet ministers will leak every last nuance of what she would do next.

In fact she has already signalled she is depriving ministers of a voice on how she will respond to the defeat - because she said she will make a statement to the Commons immediately after the vote, and there will be no time for the cabinet to meet between the vote and whatever she says.

She is thus exercising quite extraordinary personal power - which is possible because under Tory party rules she can't be booted out for at least a year, following her own MPs abortive coup just before Christmas, and because her Cabinet is united that her deal is preferable to either a no-deal Brexit or to a referendum which could keep the UK in the EU.

Also only the conceit that her deal might one day actually be approved by parliament keeps her party from splitting.

If the government ever sponsored a referendum or went for a Norway-style commercially intimate future relationship with the EU, Brexiter Tory MPs would probably break away from the party, just as Peelite Tories did over free trade in the nineteenth century; and if she adopted a managed no-deal Brexit in preference to her deal, Remainer Tories would resign the whip, and several cabinet ministers would resign.

DUP deputy leader Nigel Dodds responds to the latest statement issued on Brexit by Theresa May. Credit: PA

So the survival of her Brexit deal is crucial to maintaining the integrity of her party and her ability to stay in office - even while it alienates Northern Ireland's DUP MPs - whose support is essential for her ability to pass legislation.

One clue as to what she does next will be given by the decision she will have to make later today on whether to instruct her MPs, including ministers (the so-called payroll) on whether to support an amendment to her deal.

This would in effect instruct her to return to Brussels to negotiate for the widely hated backstop - seen by the DUP as driving a wedge between Northern Ireland and Great Britain - to expire no later than December 31, 2021.

Theresa May and European Council President Donald Tusk. Credit: PA

If that amendment only lost by a smallish number of votes, she could cite it in renewed negotiations with the EU as reason for EU leaders to surrender on the backstop - although there is a risk that as and when the amendment fails, the rest of the EU could take the opposite view, which is that it would be pointless for them to make such a humiliation capitulation if it could not guarantee a majority in the Commons for the reworked deal.

Truthfully we are only at the beginning of the end of this saga that will shape the UK's economy and standing in the world for generations.

Ultimately Tory MPs and Labour MPs - and probably independently of their leaders - will have to make a decision about whether there is a version of Brexit that unites enough of them, or whether they will have to put it back to us in a referendum.

If the existing party structures, Labour and Tory, survive intact, that will be little short of a miracle.

But the break up of our big parties may turn out to be the least of what matters.