The customs partnership option was formally floated by the Government as long ago as last August.  Why is it only now meeting such opposition among Brexiteer MPs – not only overt criticism from backbenchers, such as Jacob-Rees Mogg, but semi-covert slatings from friends of such Cabinet ministers as David Davis, Liam Fox, Michael Gove and Boris Johnson?  After all, each one of these is a member of the Government and, therefore, a critic of a policy that they themselves are signed up to, at least in outline.

The answer is that all concerned believed the idea would be dead in the water by now.  Our columnist Henry Newman, the director of Open Europe, gave it a kicking immediately after it was introduced.  It is a brilliant instance of a policy compatible with an objective in principle but not in practice – the aim in this case being for Britain to be able to negotiate and sign its own trade deals.  But why would a non-EU country have confidence in a prospective trade arrangement with another that would charge EU tariffs on its goods and then repay them later, if they were for domestic consumption?  Even more to the point, what about the non-tariff barriers?  Could prospective partners be sure that such a scheme, which hasn’t been tried anywhere else in the world, would be administered by a workable bureaucracy?  (The Windrush affair isn’t exactly boosting the reputation of UK government administration abroad.)  Newman said it might be legally challengeable.  The EU claims that the scheme is unworkable.

As we have seen during the last week, Leaver MPs are nervous about whether Theresa May will stick to her commitment to leave the Customs Union (in effect).  Their skittishness gathered pace at the end of last week, and it wasn’t until the start of this one that Downing Street offered vocal reassurance – or tried to, anyway.  In between, it was reported that “someone from the political unit at Number Ten said: ‘We wouldn’t cry into our beer if we were forced to do this.’ ” (In other words, join a customs union like the present one.)

The Commons debates the Customs Union today.  However, a vote on a motion isn’t the same thing as an amendment to a Bill: the latter might bind a Goverment’s hands; the latter does not, though it can restrict its room for manoeuvre.  This is why Ministers aren’t rushing to get the Customs Bill and the Trade Bill to the Commons soon: they know that both will provide opportunities for an amendment that would seek to commit Ministers to membership of a customs union as a negotiating objective.  But the shorter-term challenge to the Government’s unity and coherence is not a customs union but that customs partnership.  Discussion of it in Theresa May’s “war cabinet” has been postponed to next week.  The nub of the politics of the matter is whether Ministers would resign over it.  Johnson, Fox and Davis are all more likely than not to quit if the Prime Minister turns turtle on the Customs Union itself.  It is hard to see how the Government and May herself could survive such a development.

What will happen if Cabinet backs the customs partership model is more questionable.  In crude terms, the Prime Minister seems to have the backing to get it through the crucial Brexit Cabinet sub-committee on strategy and negotiations – the so-called “War Cabinet”.  And what the committee approves the Cabinet won’t block.  But rejecting the other customs option, the so-called “highly-streamed customs arrangement”, would destabilise the Cabinet further and make waves on the Conservative backbenches.

When May began to formulate her negotiation strategy before the last election, the key figure helping her to shape it was Nick Timothy.  Whatever one’s view of her former co-Chief of Staff, there is no disputing that he is a committed Brexiteer.  Since he left Number Ten after the general election debacle, there has been no Leaver of similar prominence or power within the Downing Street machine.

While the big picture framework that Timothy helped to draw up has stayed intact (the Government remains committed to leaving the EU entire, including the Single Market, Customs Union and ECJ jurisdiction) the granular detail will leave Britain, if the Prime Minister achieves her negotiating objectives, closer to the EU than might otherwise have been the case – for example over alingment and divergence.  The customs partnership model is an illustration of the shift.

The still point in this turning world is that May will be fixated on her own survival.  Over the next week, she is likely to search for a compromise that will keep the customs partnership model, or at least elements of it, in play, on the one hand, while not alienating Brexiteer Ministers, on the other.  We cheerfully admit having no idea whatsoever what this might be.  Nor, perhaps, does she.

And finally.  Following the ins and outs of policy shifts and Cabinet manouevres is political journalists’ bread and butter.  But none of these developments are taking place in a vacuum.  The formal date of Brexit is less than a year away.  And until or unless the Government resolves its position on the Customs Union, it cannot hope to make progress on key issues in the talks.  The most vivid example is the UK/Ireland border.  That makes the “backstop” position, to which Nigel Dodds took reasonable exception on this site yesterday, loom larger.

As Michel Barnier puts it, “the clock is ticking”.  Or, as a Minister puts it in a variation on the theme, “it has already passed midnight twice”.  Tick tock tick tock tick tock…