Sam McBride: Ruling out Corbyn at this stage may be a tactical mistake by the DUP and UUP

Northern Ireland’s unionist parties are united in opposing that deal, which they believe would fundamentally undermine the Union by creating a new Irish Sea border.

If Mr Johnson is not to be returned as prime minister, there is only one other credible alternative - Labour’s Jeremy Corybn.

The unionist parties are therefore hoping for Mr Corybn to either win outright, or if he falls short of an overall majority they would automatically endorse him in an attempt to block Johnson’s deal going through, right?

As anyone with even a passing knowledge of politics over recent years will realise, that is not what unionists want to happen.

The reasons are complex – and in many cases, historical. However, the stances of the DUP and the UUP could be extraordinarily short-sighted.

Unionism has long been distrustful of Mr Corbyn to the point of being contemptuous of him.

Across almost all of unionist opinion, he is seen as someone who sided not with unionists, the Alliance Party or the SDLP’s John Hume during the long years of the Troubles but repeatedly aligned himself with the representatives of the IRA – even though he insisted then as he does now that he never supported IRA violence.

Therefore the prospect of a long-standing friend of Gerry Adams becoming prime minister is understandably alarming for much of unionism.

They see Mr Corbyn as someone who could call a border poll or give the Irish government a greater role in Northern Ireland’s affairs.

In fact, since rising to the Labour leadership Mr Corbyn has generally been restrained in his policy on Northern Ireland and has even sided with unionists in opposing Theresa May’s effort to implement an Irish Sea border (although he has belatedly appeared to recant on that).

On top of that, the unionist parties are largely right of centre – although the DUP was founded to be economically left of centre – and do not share Mr Corbyn’s foreign policy, his opposition to nuclear weapons, or much of his anti-capitalist economic ideology.

But politics is about difficult choices and after this election there is unlikely to be a third way between Corbyn and Johnson.

Even for a politician, Mr Johnson has shown himself to be remarkably unreliable, publicly betraying the DUP twice in the space of six months. If in a hung parliament the DUP faces a choice – as may be the case – between backing the Tories or Labour, do they think that a leopard can change his spots? If not, both potential candidates are deeply unsatisfactory because of their past records.

Having so casually betrayed the DUP, regardless of what promises Mr Johnson makes when in such a corner, how can they ever trust him again? And yet, the DUP has been categoric that it will never put Mr Corbyn into Downing Street. In DUP literature, one of the six reasons given for voting DUP is: “For a strong DUP team who will not support a Corbyn government.”

Other DUP literature lists three headline priorities: “Defend the Union; Defeat Corbyn; Deliver Brexit”.

As on so many issues, the UUP – which looks unlikely to win seats but on an excellent day for it could come back with two MPs – has adopted an identical stance virtually identical to its larger rival.

UUP leader Steve Aiken told BBC Radio Ulster’s Talkback programme this week: “Under no circumstances will anybody from the Ulster Unionist Party be supporting Jeremy Corbyn”.   Both parties’ positions mean that in the event of a hung parliament they have left themselves little room to negotiate.

The principle that my enemy’s enemy can be my friend has been discarded.

The unionist distaste for Mr Corybn is understandable – and in many ways entirely logical.

But in a hung parliament they would be in a position to decide how long he governed.

They could support him to become prime minister in order to topple Mr Johnson and his deal and then eject him from office weeks or months later if they were unhappy at his wider programme for government.

As it is, the unionist parties – and particularly the DUP – will hope that although they have given up the ultimate threat of putting Mr Corbyn in Downing Street that they will still have the power of withholding their support from Mr Johnson and that with that power they might be able to persuade him to abandon his Brexit plans.

But that is a far weaker negotiating position than the one which Peter Robinson adopted in 2015 when he hoped for a hung parliament and resolutely refused to rule out backing a Labour government (albeit that was before Corbyn’s assent to the leadership).

The unionist position became more starkly questionable this week with the publication of the Labour manifesto.

That document commits Labour to “ensuring that there is no return to a hard border between Northern Ireland and the Republic of Ireland or the creation of a regulatory border down the Irish Sea”.

Therefore Labour is actually adopting – at least on paper, and for now – an even harder line in opposition to an Irish Sea border than the DUP, which was willing to accept a substantial regulatory border between Northern Ireland and Great Britain, only to see Mr Johnson use that concession to then push for even wider divergence.

Perhaps the DUP hopes that in a hung parliament it could force the Tories to change leader.

But, given that not a single Conservative MP opposed Mr Johnson’s deal – despite the DUP’s pleas – even that would be of limited benefit on this issue.

The DUP is renowned for its ruthless pragmatism. But, having become close to the Tories over the last two years, the party’s response to the way in which it has twice been abandoned by Mr Johnson has been uncharacteristically restrained.

There has long been a Conservative view that the DUP would never put Mr Corbyn in Downing Street and therefore it was unnecessary for the Tories to do so much – especially in agreeing the £1 billion confidence and supply deal – to court Arlene Foster.

The DUP’s refusal to countenance supporting Mr Corybyn – even after being so shabbily treated by Mr Johnson – appears to bear out that hypothesis.