It turns out there is something Northern Ireland’s Democratic Unionist Party fears and loathes more than the possibility of a government led by Jeremy Corbyn.
They would be prepared to sink Theresa May and her government to prevent even the remotest prospect of a border being introduced in the middle of the Irish Sea between Northern Ireland and the rest of the UK.
Which is why, as I said earlier, the prime minister has to be quadruply clear that any regulatory alignment she offers to the EU to prevent the re-establishment of a border between Northern Ireland and the Republic has to be alignment that applies clearly and equitably to the whole of the UK and not just to Northern Ireland.
The point is that alignment of just Northern Ireland - even if restricted to the narrow “Belfast Agreement” economic sectors of agriculture, energy, transport and veterinary products and services (though none of these are in any meaningful sense in the actual Belfast Agreement) - would establish a border in the sea.
And the reason is that if British regulations were different from Northern Irish ones, there would have to be border checks when Northern Ireland stuff was shipped to England and vice versa.
So that’s only going to happen over the DUP’s dead body. Or rather, they would say ruefully, over Theresa May’s dead body.
But there is a problem with regulatory convergence between the whole of the UK and the EU - it does not look like "taking back control", to coin the favourite phrase of Boris Johnson and Michael Gove.
So they aren’t thrilled by it.
And it is economically irrational for the alignment to apply only to agriculture, energy and so on, rather than other industries like motor manufacturing - which has been crying out for just such regulatory convergence as a facilitator of seamless, costless trade.
So tomorrow she will have to give her Cabinet the hard truth that a Brexit accompanied by an economically benign trade deal will require a degree of permanent economic convergence between the UK and EU which for a Gove or Johnson would negate much of the point of Brexit.
But if they opt for a no-deal Brexit, Parliament will probably veto any Brexit at all. It’s a mess.
And it is on nights like these when the PM probably remembers precisely why she voted Remain.