Labour has talked itself into a strange place on the Irish border. Since the publication of the draft withdrawal agreement last week, Jeremy Corbyn has – to increasing consternation on his own benches – repeatedly attacked the backstop in terms one would expect to hear from the DUP or European Research Group.

Best understood as an EU insurance policy for avoiding a hard border on the island of Ireland, the backstop will see Northern Ireland remain in a deeper customs union with Europe than the rest of the United Kingdom. It will also see single market regulations apply to Northern Ireland alone, necessitating new checks in the Irish Sea (or a border, if you want to call it that). The UK will not be able to leave unilaterally.

By the DUP’s uncompromising unionist logic, this is anathema and an affront to the constitutional integrity of the United Kingdom. Brexiteers who have quit or otherwise oppose the government, keen to demonstrate that May’s plans have no majority, are saying they cannot support the deal for the same reasons.

That the DUP and ERG are saying this isn’t all that surprising. Team Corbyn’s decision to parrot uncritically those attack lines is. Tactically, it makes basic sense: at every available opportunity since the deal was published last week, he has punched the bruise that will cause Theresa May the most political pain as far as trying to assemble a majority for her withdrawal agreement goes. Every time he asks a question along these lines in the Commons, the prime minister is forced to recite an answer that enrages the Conservative and DUP MPs she needs on side to pass anything.

It makes sense – and then, very quickly, it doesn’t. Strategically, it's myopic. If you follow the setlist of Corbyn’s Arlene Foster tribute act to its logical closing number, it follows that a Labour government would not countenance anything that resulted in a “border down the Irish Sea”. For political purposes, Corbyn is defining “border down the Irish Sea” as the new regulatory checks for goods entering Northern Ireland from Great Britain provided for in the backstop agreement the government has signed up to. It’s an absolutist position, is at odds with the stance of his Northern Irish sister party and most businesses in Northern Ireland, and sharply limits Labour’s room to propose an alternative.

In fact, it arguably means that Labour could not accept a backstop at all. The immediate and most significant consequence of this is that you can’t have a withdrawal agreement at all, let alone a jobs-first trade agreement or whatever.

How, then, does Labour square its opposition to a border in the Irish Sea with its claims that it is ready and waiting to negotiate a better Brexit than Theresa May? Asked about this after PMQs today, Corbyn’s spokesman said – correctly – that the permanent customs union that Labour wants would “mostly” remove the need for the imposition of border checks, ergo there is no need for a backstop as it’s already sorted.

Where this argument falls down is at the point the word “mostly” appears. A customs union cannot solve the border issue by itself – if it did, the government would not be having the confrontation with the DUP it is currently having. There must be regulatory alignment between Northern Ireland and the Republic on top of this, hence the need for what Corbyn and his unlikely bedfellows are calling “a border in the Irish Sea”.

Agreeing a permanent customs union with the EU would still see a UK government face a scenario where it had to agree to separate regulatory provisions for Northern Ireland alone. So, again, we return to the same question: if Labour’s customs union doesn’t remove the need for a backstop by itself, but a backstop is unacceptable, how are they going to sign a withdrawal agreement? The answer Corbyn’s spokesman gave was sketchy, but the nub of it was that they did not anticipate that their withdrawal agreement would need a backstop on the grounds that their customs union and plan to align closely on most regulations would obviate the need for it.

That line appears to betray a misunderstanding of what the backstop is – an essential precondition for finalising the divorce; and of why the EU are insisting on it being so – to safeguard Ireland against future changes in UK government policy. The shape of the future trade relationship that Labour wants to negotiate is by the by: there will have to be a backstop in Labour’s withdrawal agreement unless they are suggesting the reunification of Ireland or special status for Northern Ireland (which they’re not). Even if they did not intend to ever enter the backstop, it would still be there.

If they really do anticipate they won’t need one, they are anticipating wrongly. However politically expedient it is, by suggesting otherwise it is either being disingenuous or stupid. Or quite possibly both.