Sealed by the King, the Windsor Framework is the DUP's worst nightmare.
Northern Ireland's flag-waving hardline Unionists reject the proposed deal and backs are turned on the monarchy they worship.
To apply the Stormont Brake on fresh EU laws these same Unionists must resume power-sharing in the Belfast Assembly.
And that involves accepting a First Minister from bitter rival Sinn Fein, a republican party dreaming of one Ireland and living within the EU.
There's a touch of ingenuity in Rishi Sunak's scheme, putting the DUP and his own party's loopy Brextremist enemies on the backfoot.
Victory may yet fail to strengthen a weak Prime Minister, the cost of winning another front in the Conservative sapping civil war.
Defeat's still a possibility, with a shameless Boris Johnson scheming to be reinstalled to No 10 plotting against a deal which is a marked improvement on his own rotten, dishonest 2020 protocol.
Ultimate triumph might require a review of the 25-year-old Good Friday Agreement to abolish a DUP veto.
And of course Sunak should've thought about Northern Ireland's 310-mile UK land border with the Republic and EU when ignorantly advocating Brexit in 2016.
But it's important to solve a trade problem and reset relations with neighbours for the UK to participate fully in the crucial Horizon Europe scientific research programme.
So credit where it's due when constructive Sunak's on the cusp of an agreement that wrecker Johnson could never reach.
British membership of the EU remains the best deal and Keir Starmer's Labour votes may help get this over the line in Parliament.
Nor must we forget Northern Ireland voted to remain in Europe and most accept continued EU involvement in a corner of the UK the DUP doesn't speak for.
This is a fight between common sense and pragmatism against “no surrender” zealots.
