I’ve been saying for days that provision in NI Brexit backstop keeping UK in customs union would be "temporary" but would not have a legally binding end point. A negotiated Brexit, and Theresa May's survival, hinge on whether her ministers and MPs can live with this paradox.
Oliver Robbins and May will try to reconcile this contradiction by persuading the rest of the EU to sign up in the separate political declaration that a future commercial relationship between the UK and the EU will deliver "frictionless, tariff-free trade" between UK and EU.
If they succeeded this would mean backstop would become redundant, because border between NI and ROI would be kept open by other means than membership of customs union.
But...there are two substantial defects with this cunning plan.
First, the EU will never give a blanket pledge to deliver an end-state to the UK as beneficial as being in customs union without UK adhering to all the conditions of customs-union membership (the ones May and her government spurn, like a ban on doing free trade deals with other countries).
Second even if the EU wrote weasel words in political declaration that seemed to promise something like "frictionless tariff-free trade", the political declaration is simply a statement of intent.
It is NOT a legally binding document.
So all that would happen if we signed backstop keeping us in customs union sine die is that the ghastly debate about whether ultimately to go for a hard rupture with the EU, a hard Brexit, would go on for year after confidence-eroding year.
Thus the hugely difficult assessment for Brexiters in the Cabinet is whether they are being stitched up as the UK is forced into permanent subjugation to the feudal overlords of Brussels, which would indeed be a reason to resign, or whether the PM’s judgement is essentially correct, that it is impossible to settle all practicalities of the UK’s trading relationship with the EU this side of March 29 2019.
The better side of valour is to continue the fight for putative Brexit liberation for years yet.
So in the coming days, when we see legal text of draft backstop and Withdrawal Agreement, this will surely be the time for Tory Brexiters to choose between supporting the PM and maintaining the fiction of party unity on the one hand and their ambitions for what the UK could and should be.