It would be the crowning achievement of Peter Robinson to unite unionism but, like David Cameron before him, the DUP leader may find the UUP a shrewish partner, hard to please and changeable in mood.

There has been talk of friendly overtures between the two parties for some weeks. Robinson gave wings to the rumours in an interview with UTV's Ken Reid on Tuesday.

"You may have noted, over the past couple of months, some of the heat that has been in the exchanges between the DUP and UUP has now been removed," he said, speaking of his good relationship with Tom Elliott, the UUP leader, and Danny Kennedy, its minister.

Robinson proposed united opposition to Scottish independence, an echo of the opposition to Irish Home Rule, which united unionism exactly a hundred years ago.

The DUP leader showed some ankle, adding coquettishly that "what flows from a better relationship only time will tell".

In a sense, he was saying nothing new. He has long made it clear that he would like to see unionism united; he regards it as a legacy issue.

The problem is that nothing divides unionism like the attempt to unite it. That has been the case ever since Ian Paisley, Robinson's predecessor, split unionism in the first place.

That left bitter memories among older UUP members, who blame Paisley for destroying the unionist monolith which dominated local politics for most of our history.

The UUP is not a cohesive party, or, putting the same thought more positively, it is a broad church which values diversity and a hasty merger would almost certainly split it.

The bulk of members might go to the DUP, but the local Tories and Alliance would also stand to benefit and the emergence of a new party could not be ruled out.

The DUP is a more united body, which contains any differences within its ranks by dint of iron discipline and the judicious distribution of patronage.

MLAs who sound off without clearance are punished. It happened to Jim Wells for a period after the DUP agreed to go into power with Sinn Fein and he described what had happened at the meeting where the vote was taken. In the UUP, talking after a meeting would hardly merit a rebuke.

To be fair, the DUP has valuable experience of making former UUP members, like Jeffrey Donaldson and Arlene Foster, feel at home. However, their defections also left a legacy of distrust in the UUP.

In Tom Elliott's Fermanagh Unionist association, for instance, there is still resentment at Ms Foster for leaving. The UUP also blames the DUP for backing Jim Dixon, an independent, in the 2001 Westminster election. The result was that Michelle Gildernew of Sinn Fein won the seat by a handful of votes over the UUP's James Cooper.

There is a lot of history between these two parties, much of it poisoned. It will take all of Robinson's legendary strategic abilities to bring this particular mission to a successful conclusion.