Boris Johnson told Novembers DUP conference what it wanted to hear but this week he abandoned the DUP

The unity between the DUP and the right wing of the Conservative Party has been more than simply a marriage of convenience.

Whether on unionism or social policy, the DUP has always felt closest to Tory right-wingers in Parliament and for MPs such as Sammy Wilson and Ian Paisley Jr, it is the ERG which most closely represents their vision of Brexit.

But when at the last both sides were forced to choose, there was always going to be an inherent tension. For many in the ERG, while they are pro-Union, leaving the EU has for many years been their central political goal.

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But for the DUP, while they are – overwhelmingly, if not uniformly – pro-Brexit, the central tenet of their political philosophy is protecting Northern Ireland’s place within the Union.

Until recently, both sides of the alliance believed that they did not have to choose between which of those beliefs was most important because they could either secure concessions from the EU which would neuter or make less unpalatable the Northern Irish backstop, or else leave the EU with no deal.

It is the backstop – which would treat Northern Ireland differently to the rest of the UK, and out of which there would be no unilateral exit – which both the DUP and ERG agreed was a threat to the Union, opening the potential for Northern Ireland to diverge over time from the rest of the UK, while having no democratic say in the EU rules by which it would be bound.

Had the backstop been altered it in a meaningful way, the DUP and ERG would likely have continued to act as a bloc.

But, for now at least, it seems clear that there will be no EU concessions. The DUP was dismayed that at last week’s EU summit Theresa May did not even ask for significant changes. They believed – with some justification – that it was only when the EU began to believe that the UK might actually leave with no deal that the strains within the EU began to emerge.

The DUP, a party of transactional politics which is keener to drive a hard bargain than to build a lasting relationship, saw last week’s summit as the moment when Mrs May could have secured something, had she the steel or the nous to do so.

The prime minister appears to have accepted that there was no point in wasting time trying to reopen what the EU has said was a closed document and instead focussed on getting a temporary delay to Brexit to prevent a sudden no-deal departure.

But the DUP leadership has continued to talk to the government – with some of those talks focussing on an acceptance of the backstop, but with Stormont having a role in how it would operate. That shows that even at this late stage the DUP, a party which trumpets principle but is deeply pragmatic, is open to further compromise.

However, with council elections looming and with Arlene Foster in a weak position where she needs a strong electoral endorsement, it would be a huge risk to do the sort of handbrake turn performed by Boris Johnson this week.

There was little thought for the DUP from Mr Johnson when he abandoned a pledge which he gave to the DUP conference just four months ago. At that point he had said that May’s deal would mean Northern Ireland becoming “an economic semi-colony of the EU”, adding that “no Conservative British Government could or should sign up to such arrangements”.

Within minutes of Mrs May making clear that she would quit if her deal passed, thus opening up a Tory leadership election in which Mr Johnson would be a favourite, he said that “though it fills me with pain, I’m going to have to support this thing”.

But the previous evening, as Tory Brexiteers began to U-turn, Richard Bullick – who for 17 years was a key brain behind the New DUP until 2017 – tweeted the sort of calculation which will have been considered within the party.

He said: “Have none of the ERG switchers realised that if the backstop was agreed to by Parliament the best interests of unionism would be served by a much softer Brexit than they support?”

The DUP has always laid out just one red line on Brexit: Treat Northern Ireland the same as the rest of the UK. The ineluctable logic of that position has always been that everything else is up for negotiation.

Therefore, while the DUP’s ideological preference is for a harder form of Brexit – leaving the Customs Union and the Single Market – those have never been red lines. If pursuing that cleaner break with the EU was only to come at the expense of the integrity of the UK, it follows that a soft Brexit would satisfy the DUP’s only firm demand.

The DUP has not become Northern Ireland’s largest political party through naive reliance on those who publicly claim to have its interests at heart. Many in the DUP knew that they could not count on some ERG members – and particularly Mr Johnson. Rather, it suited both sides to use each other, in the full knowledge that a dagger might have to be plunged into the partner at some later point.

While Mr Johnson’s speech at the DUP conference was greeted with roars of assent and hoots of warm laughter, some of the DUP members hollering the loudest privately saw him as an untrustworthy opportunist who could not be relied upon to stand by the DUP when it really counted. At the end of his speech, I asked one DUP member if he trusted Johnson. He laughed knowingly and replied: “Well, he’s a politician, isn’t he?”

In an interview with the BBC’s Mark Devenport on Thursday night, Nigel Dodds made explicit the DUP’s position, saying that it was possible we are now heading for either no Brexit or a much softer form of Brexit. However, he said that the DUP’s concern “isn’t necessarily the form of Brexit; it’s making sure that in whatever form of Brexit we have that those trade barriers between ourselves and our main market in the rest of the United Kingdom and the constitutional issues about who makes our laws and all of that...are protected.”

But, even before Mr Dodds’ more explicit comments last night, the logic of the DUP’s position has always extended to no Brexit at all if it is the only way of keeping the Union intact. Having liked the idea of Brexit but never having been doctrinaire about its form, the DUP’s one red line means that if there is a choice between staying in the EU and maintaining the constitutional integrity of the UK or leaving in a way which weakens the Union, then the DUP was always bound to endorse remaining in the EU.

We are not yet at that point and it may not arise. But if that becomes the choice then the DUP will be unsentimental about such a radical ideological shift. And, unlike Mr Johnson, they have arguably been more upfront about that possibility.

But the existential constitutional threats with which the DUP is now grappling reinforce the risk which the party took to back Brexit with no clear plan for how to implement it.