Analysis: Is the UUP going the way of the SDLP, asks Gerry Moriarty, Northern Editor.
On the first day of this month it was written here that after the Assembly elections the logic of Mr Jeffrey Donaldson resigning from the Ulster Unionist Party would be "never stronger". Logic has never had much place in Northern Ireland politics but it did yesterday - Jeffrey has finally jumped.
The Lagan Valley MLA defected with Assembly colleagues Norah Beare and Arlene Foster. It was an angry leaving, not a gracious one. As far as Ulster unionism is concerned, this is the season of ill will. "I am not going to give David Trimble and his cronies the joy of throwing me out of the party," he told UTV.
First the resignation, later the likely switch to the DUP. Mr Donaldson indicated that he would have no difficulty serving under the Rev Ian Paisley, and the DUP in turn welcomed the move, offering him a position on its agreement review team.
Mr David Trimble issued a measured statement of response last night, but be certain that he was neither surprised nor saddened. There certainly was an ideological basis to the Trimble-Donaldson divisions within the UUP, but much of the differences were personal.
The depth of the animosity was captured by a comment by a pro-agreement Ulster Unionist. Asked by The Irish Times ahead of the announcement what he thought of Mr Donaldson's decision, he replied, "Oh, has he moved? Well, thank God for that. Christmas has come early."
Since the Belfast Agreement was signed over five years ago on Good Friday 1998, Mr Donaldson's influence on Mr Trimble and the Yes forces of the party was always debilitating, even at times when the UUP had a chance of overcoming its internal and external unionist enemies.
Many remember all those Ulster Unionist Council heaves, all those close victories for Mr Trimble over Mr Donaldson, all those verbal spats that gradually undermined the power and prestige of the UUP.
Therefore one can allow for a little badmouthing of Mr Donaldson from the Trimble camp. But any engagement in spite had better be brief because this move by Mr Donaldson is of huge significance. The move by Mr Donaldson, Ms Foster and Ms Beare creates the opportunity for the DUP to do to the UUP what Sinn Féin is hoping to do to the SDLP.
Sinn Féin believes it is on a roll as far as the SDLP is concerned and that in forthcoming elections Mr Mark Durkan's party will be relegated to the position of also-rans. It's for Mr Durkan and his colleagues to resist that threat.
Equally Mr Donaldson spoke of a "realignment within unionism". The Lagan Valley MP knows his resignation is a gamble. He is ambitious. By biding his time he could have been the next leader of Ulster Unionism.
But Mr Donaldson has judged that the UUP is not worth the wait, and that's a cruel judgment. If he is correct, the Ulster Unionist Party will end up chasing the DUP just as the SDLP is now chasing Sinn Féin. He could topple a monolith that has survived since the turn of the century, albeit a crumbling monolith in recent years.
Trimbleites say that his resignation will clear the air, that the party will now be able to campaign on a unified platform, in support of the agreement but placing the spotlight on republicans and demanding that they deliver the "acts of completion".
That's a noble unionist objective but the danger as far as Mr Trimble is concerned is that such is the speed of the wind behind the DUP that responsibility for that project will pass by some sort of osmosis to the DUP.
Into this dangerous mix must be added the names of anti-agreement Ulster Unionists Mr David Burnside and the Rev Martin Smyth, who so far are remaining within the UUP. Now that Mr Donaldson has left they won't be as strong, but they can still grab those headlines.
Last night, the Rev Ian Paisley and the DUP tacticians were rejoicing. Dr Paisley has always hated what he sees as Big House Unionism. In his twilight years is he witnessing its demise?
What a year for Dr Paisley. His negotiating hand in the New Year review of the Belfast Agreement is strengthened. Even in a loose alliance with Mr Donaldson, he will have a negotiating team of 33 against 24 for the UUP, with Mr Robert McCartney and Mr David Ervine serving as pro- and anti-agreement additions.
So much has changed since the Assembly elections. The main protagonists are now Dr Paisley and Mr Gerry Adams. The future of the Belfast Agreement and power-sharing and some form of essential community consensus rests on whether politicians so diametrically opposed can find some form of middle ground.
Were that possible - and some DUP pragmatists insist it is - then Mr Donaldson and his colleagues must join the DUP. Mr Donaldson by moving to the DUP would be assured of a ministry.
Could all that power be the spur for the DUP and the Donaldson rebels to eventually thrash out a deal with Sinn Féin and the IRA that the moderates in the SDLP and UUP couldn't? That's for the New Year.
In the meantime, Mr Donaldson's defection has further radically altered the face of Northern Ireland politics.