The two main unionist parties have perhaps never been closer ideologically than they are now, but this week saw a development that will drive them further apart than they have been before.
Relations between the Ulster Unionist Party and the DUP have waxed and waned since the latter’s formation in 1971, and got very close in the aftermath of the 1985 Anglo Irish Agreement, even though they were in spirit quite different organisations then (with the Rev Dr Ian Paisley, at his most firebrand, still at the helm of the latter).
Now many of the most senior members of the DUP are ex Ulster Unionists (Arlene Foster, Simon Hamilton, Jeffrey Donaldson, William Humphrey), and the two parties’ policies are barely distinguishable.
Both have accepted power sharing, albeit with varying degrees of enthusiasm, both are slow to criticise the big state (in all its manifestations such as free prescriptions and reluctance to reform hospital provision and other aspects of public expenditure), and both try to avoid contentious social issues because many of their younger members lean liberal on such topics but most of their older members lean conservative.
You might think then that it would be sensible for the two parties to merge, particularly given that there has been a new flank in unionism, the Traditional Unionist Voice, for the best part of a decade.
Its emergence in 2007, when Jim Allister quit the DUP over Sinn Fein in government, was the most recent schism in unionism. His TUV occupies the ideological space where the DUP once was.
A further unionist flank emerged when NI21 was formed in 2013, but that moderate movement flopped.
Despite this crowded provision of unionism, the two big unionist parties moved further apart this month when Mike Nesbitt took the UUP into opposition.
It seemed that this might leave them isolated, but suddenly on Thursday a centrist grouping came together when the SDLP and Alliance decided that they too would go into opposition (although it was unclear last night if Alliance will stay out of power).
This is a seismic development. For the first time since 1998, perhaps even 1974, the SDLP and Ulster Unionists are looking more towards each other for occasional support than towards the other big party on the same side of the community divide.
But there is a difference between now and both 1974 and 1998. This time, unlike then, there is not within the Ulster Unionist Party a large minority that is deeply unhappy with the momentous decision it has taken. I talked to several senior party members yesterday, and there seems to be broad enthusiasm for Mike Nesbitt’s move.
Half the UUP’s 16 MLAs are new and they are particularly pro going into opposition.
Half the SDLP’s 12 MLAs are also newly elected and they too are said to be keen on Colum Eastwood’s decision.
The Alliance rank and file membership is reported to be similarly enthusiastic on the party going into opposition.
While both the SDLP and UUP have at times seemed to try to out-Green or out-Orange their rivals, SF and the DUP, there is nonetheless a fundamental logic to their move towards working together. The SDLP and UUP are now perhaps down to a core vote that comprises many moderates – the people who are least likely to want to move closer to their more emphatically nationalist or unionist parties.
There is a wider logic too. For all the undoubted success of Sinn Fein and particularly the DUP at the polls this month, there is simultaneously a gradual shift away from tribal politics. The ‘others’ vote (neither unionist nor nationalist) keeps rising in elections, albeit from a small base.
The combined DUP-SF vote this time was 369,352, far out in front of any other combination.
But the combined SDLP-Green Party-Alliance-UUP vote was nonetheless significant, at 237,831.
If People Before Profit and Tories and Labour and moderate independents such as John McCallister are added to the latter total, it rises to over 270,000.
There are also the minor unionist parties such as TUV and Ukip and PUP, that bring the ‘others’ total to around 320,000. Those parties could hardly be described as moderate, but they are in no mood to work with the SF-DUP pact, and so they too fall into the others category.
In summary, the DUP and SF are still in front, but the opposition to them is considerable, a 53-47 split.
When you consider the determination of people as diverse as Eamonn McCann and Jim Allister to challenge the executive, Arlene Foster and Martin McGuinness will face a wide front of opposition, both formal and not.
Perhaps opposition will bolster the centre ground, which in any event should be rising given the growing numbers of people who do not identify themselves principally under a tribal label.
Or perhaps it will bolster Sinn Fein and the DUP as the mainstream parties of governance.
The DUP said yesterday that the SDLP and UUP had gone into opposition because they had had their worst ever results. There is some truth in that. But the DUP was at times vicious about the record of UUP ministers, which was hardly going to make the latter feel warmly about coalition.
In any event a vast rainbow coalition executive never looked sustainable, with the absurdity of ministers sniping at each other and even suing departments of ministerial colleagues.
It seemed that there was no way out of the rainbow coalition, until John McCallister provided a way with his opposition bill. John lost his South Down seat and is not around to savour that achievement.
He was one of Stormont’s most interesting figures, challenging populism on matters ranging from recycling (which he showed could be pushed further) to public sector reform (about which he felt Stormont was not doing enough).
Ben Lowry (@BenLowry2) is News Letter deputy editor