Northern Ireland, by some informed accounts, could be standing on the brink of a constitutional change of the significance of the 1985 Anglo Irish agreement.
The DUP certainly seems to fear that something big is happening, because it has made clear that it will vote against the government in key divisions — perhaps including the budget — if a border is introduced in the Irish Sea.
The threat seems to be a regulatory border, which sounds harmless enough (invisible checks at sea and so on) but in fact would have huge ramifications for the Province.
Northern Ireland would never again be in the internal UK market, and would forever be in the EU one.
That would be a major breach in our place in the UK.
Thus the DUP has no option but to make clear that the confidence and supply agreement is in jeopardy.
This in turn has soured relations between the parties.
It all makes an election and a Jeremy Corbyn premiership more likely but if the Tories are going to introduce a change as significant as anything the Labour leader might do then unionists are in a bind, and must stick to core principles.
As Boris Johnson writes on these pages, this all goes back to the disastrous backstop of last December. It clearly needs to go, but the problem is the campaign to scrap it should have begun months ago.
It is not too late for Downing Street to pull back from this madness.