Imagine for a moment that, at some point before Christmas or in the New Year, the backstop was radically amended – that a unilateral exit mechanism were to be slapped on to it. Then go on to picture the following events. The revised deal passes through Parliament, and there is no early general election. And at the end of transition in 2020, a decision is taken about whether or not to extend it. Let’s presume that at this point it is indeed lengthened for, say, six months. At this point, the backstop duly kicks in. Then, at the beginning of 2022, the Government leaves the backstop, just in time for the general election of later that year.

During this sequence of events, the first substantial objection to the backstop – that we aren’t free to leave it – would have fallen away this very winter. The second complaint – that the backstop places a regulatory border in the Irish sea and, given the presence of a backstop within a backstop in the text of the Withdrawal Agreement, a potential customs border too – would last for a mere sixth months or so. Then the division that it causes within the United Kingdom would end.

From that point, the Government would have the freedom to leave the continuing customs union, and negotiate, sign and effect meaningful trade deals. For example, the way would be open for it to pursue one with the United States, if it wished: the American Government has made it clear that any such arrangement will be restricted if the backstop is in place. More broadly, the gains for Britain from May’s deal would at this point no longer be outweighed by the losses.

Let us remind ourselves what these wins would be. As we wrote in our analysis of the deal after it was agreed, we would regain control of our borders, under its proposed terms, after the end of the transition period, extended or unextended. Free movement would be no more. We would also regain control of money. We would almost certainly want to pay up for participation in specific European programmes. But automatic payments into the EU budget would come to an end. A Prime Minister Johnson would be able to tip the entire sum down the gaping maw of the NHS if Parliament so agreed.

There would be a role for the European Court of Justice in relation to EU nationals for eight years. An arbitration panel would refer points of EU law to the Court, and there is a good case for saying that the panel would then be bound by its rulings. But it is important not to confuse disputes about the meaning of EU law with those between the UK and the EU more broadly. These would be resolved by a dispute resolution process. Meanwhile, the Withdrawal Agreement’s legal underpinning for the backstop would become otiose when the backstop ended.

In short, Theresa May won on borders and money in the negotiation, and minimised the ECJ’s scope on laws, which could reasonably be scored as a points win. She has won almost no credit for this achievement, first, because she has no media allies or strong public backing, but faces formidable opposition from both second referendum Remainers and UKIP-type Leavers; second, because U-turns and broken pledges elsewhere have bust her credibility and third, of course, because of the backstop.

There is another, big, structural gain from her deal. Under its terms, we would be tied to developing EU acquis on state aid and competition. However, we would not be so bound elsewhere – for example, in social or environmental matters. Some EU27 countries are worried that British governments will be able to undercut their social model in future. So under the deal’s terms, we will gain freedom of movement for goods – up to a point – without conceding freedom of movement for peoples. The four freedoms have been prised apart.

Now, there are powerful objections to this rosy scenario. Frictionless entry to EU for British goods will doubtless be bargained off against permissive entry to the UK for EU citizens, and to British waters for EU fishermen. Guarantees in the Political Declaration rather than the Withdrawal Agreement lack legal bite. Even were a unilateral exit to be negotiated from the backstop, we would still have agreed to pay the £39 billion or more agreed under the terms of the agreement – thus reducing our bargaining power during trade negotiations. Essentially, the EU wants a high-alignment, high-access settlement, and so does our Treasury-led Government.

And the strongest case against our imaginary dropping of a permanent backstop is that it simply isn’t on the table: that the EU will not shaft a remaining member, Ireland, for the sake of a departing one, the UK, as this morning’s news confirms. We concede the point at once. Were Leo Varadkar not determined to take a high-risk gamble that the UK/Ireland land border will end up no harder than now – and had the Government rumbled him earlier and treated Ireland more attentively – matters might not have reached this pass. Still, we are where we are. This Prime Minister is most unlikely to win any worthwhile backstop concession in the New Year.

Why sketch out this scenario at all, then, if it almost certainly won’t happen? The answer is: to make a point well worth making – namely, that only a single obstacle prevents May from winning the backing of her Party for her deal. Most of the hostility to it would collapse were there a uniteral exit mechanism. The list of objectors would then shrink from the 71 we clocked to a much smaller number: fewer than 20, at a guess. Most would swallow a limited role for the ECJ, and reject the other objections that we have listed.

Sure, they would say, the EU will seek to gain entry for their citizens and fishermen. But it would have no automatic right to either – and that’s what the referendum was all about, wasn’t it? – taking back control. Yes, we would have lose some bargaining power by agreeing to part with £39 billion. None the less, we would retain some too, because of our power to refuse access to our country and waters. All in all, a reformed backstop would be allow the Conservatives and Labour to square off against each other on EU policy in future elections.

For were the UK free of the backstop come 2022, the Tory election manifesto would reflect its Eurosceptic centre of gravity, by proposing a Canada-type policy for future trade talks. Labour’s, meanwhile, would be more Norwegian in flavour. These two visions would then compete at the polls – at least to the degree that both parties, and voters, wanted to fix their attention on the future of Brexit.

As we say, this won’t happen – at least under this Prime Minister. Her deal and the backstop march together in step. And admittedly, even with a right to unilateral exit, this Government would be unlikely to exercise it if no deal waited on the other side of the door.

None the less, that exit would be there – which, ultimately, is what matters. We’ve said before that Brexit isn’t a still photo, but a moving film – or should be. Where Britain will be on day one isn’t where we will be in year ten. The backstop freezes that film and prevents it from playing. Provide a sure means of escape from it, and the film begins to roll. And May’s deal thus becomes acceptable.

Unfortunately, there is vanishingly little prospect of that. The backstop lies between her and success like a hollow in the path of a runner. It is so narrow as almost to be leapable. But it plunges many, many miles deep.