One of the biggest strategic errors made by any participant in the 2017 election was the failure of Remain parties in Northern Ireland to agree electoral pacts.

While an appetite for cooperation existed — and might, on a good night, have kept the DUP to eight rather than 10 seats — politics intruded.

The SDLP failed to strike a deal with Alliance and Sinn Féin in South Belfast, and lost the most diverse — and heavily Remain — seat to the DUP’s Emma Little-Pengelly as a result. Neither party was willing to stand aside for Alasdair McDonnell, the divisive incumbent, due to his anti-abortion stance (among other things).

In North Belfast, meanwhile, cooperation between pro-EU parties might conceivably have unseated Nigel Dodds, the leader of the DUP at Westminster. In second place, however, is Sinn Féin — whose abstentionism tendered any pact with their fellow nationalists in the SDLP a non-starter.

In 2017, the result of the impasse was wipeout for the SDLP – who lost all three of their MPs – and a parliament in which the DUP not only held the whip hand, but spoke almost exclusively for Northern Ireland.

So it is striking to see the SDLP voluntarily withdraw from North Belfast for the benefit of a party to which it owes nothing — except, perhaps, for its precipitous decline. Nichola Mallon, the party’s deputy leader and the likeliest candidate to have run against Dodds and Sinn Féin’s John Finucane, justifies their decision thus: “The scale of the threat we are facing demands bold and definitive action even where that is unprecedented and uncomfortable for us.”

To that end, the party will also stand aside for the benefit of Remain candidates in North Down – where independent unionist Sylvia Hermon is only 1,208 votes ahead of the DUP – and East Belfast, where Alliance leader Naomi Long is seeking to regain the seat she held from 2010 to 2015 from the DUP’s Gavin Robinson. In none of the three seats did the SDLP retain its deposit in 2017: indeed, its combined tally in Hermon’s and Robinson’s seats was just 567 votes. The 2,058 it won in North Belfast, however, is nearly equal to DUP’s 2,081 majority.

The brute fact of the arithmetic, then, is clear. And in one respect, so are the politics: a quid pro quo has come in the form of Sinn Féin’s withdrawal from Belfast South, where the SDLP’s Claire Hanna is second to the DUP (they have also stood aside in East Belfast and North Down). Some 70 per cent of Sinn Féin’s second preferences went to the SDLP in May’s local elections, and the party clearly believes that a similar windfall will come in December. Mary Lou McDonald, the Sinn Féin leader, has endorsed Hanna – as well as Long and Hermon. (It is worth asking whether their rather surreal endorsement of Hermon, the widow of a former chief constable of the Royal Ulster Constabulary, is good news for a unionist candidate attempting to stave off the advance of the DUP.)

Yet the arrangement poses as many questions as it does provide electoral answers for the SDLP. The first, and biggest, is why it is standing aside for a candidate who, if elected, will not take his seat in Westminster.

Even a sympathetic reader of the situation would have to concede that it is a fundamentally incoherent position for a party that has always made a virtue of turning up in the Commons. Doubly so, given its leader, Colum Eastwood, will be attempting to win back Foyle from Sinn Féin while effectively endorsing an abstentionist in North Belfast. Even his supporters privately admit he has made his own job harder.

Set against this backdrop, it is unsurprising that Alliance, who as a self-styled cross-community party make much of their opposition to pacts, sense they might end up the real beneficiaries of the arrangements the moderate parties of unionism and nationalism have struck in recent days.

Though the SDLP will point to their decision to stand aside for a unionist in Hermon and a cross-community candidate in Long as a defence against claims of a sectarian carve-up, that is the charge already being levelled by Alliance – who themselves believe they can win in South Belfast. And that poses a considerable risk not just for the SDLP and Sinn Féin but for the pro-European cause they claim to be serving.

The gamble, as with any electoral pact, is that voters are biddable and broadly motivated by the same thing in the polling booth – in this case, wanting to see a Remain candidate win regardless of their partisan baggage. Today's news puts Hanna at a clear advantage: it is now even trickier to argue with her claim to be the strongest Remain candidate, and much easier for her to squeeze the 2017 Alliance vote.

But if SDLP voters in North and South Belfast – or, in the latter case, those who have historically lent the SDLP their votes – are discomfited enough to respond to this effective endorsement of Sinn Féin by opting instead for Alliance, the net result may well be that two DUP MPs survive. Ditto if a Sinn Féin endorsement hurts Hermon, as could well happen. Though the odds appear to have swung decisively in the favour of Remainers of all hues, there is a slim but existent chance that this attempt to right the strategic wrongs of the Remain approach in 2017 might yet produce a similar result.