The stakes in the battle for the heart and soul of unionism were further raised last night when Jeffrey Donaldson MP finally committed himself to a No vote in next Friday's referendum on the Belfast agreement.
For Mr Donaldson's personal decision has potentially huge implications in terms of its impact on as-yet-undecided unionist voters; the declared policy position of the Ulster Unionist Party in relation to the agreement; and even a possible future realignment within unionism, should Mr David Trimble fail to carry a convincing number of unionists with him next Friday and in the subsequent assembly elections.
Mr Donaldson stopped short of explicitly encouraging the wider unionist electorate to follow his example. But the formulation of words was reminiscent of the kind often employed by his predecessor and mentor Lord Molyneaux, and such that the absence of a firm injunction seemed neither here nor there. The import and effect was the same. And Mr Donaldson later confirmed there was no prospect of him converting to the Yes camp "on the basis that the Prime Minister is unable to deliver the assurances we seek because the agreement ties his hands."
It was around lunchtime yesterday when the first signals appeared that the Lagan Valley MP had decided to put an end to his personal agonising.
Over breakfast some hours before, he had shrugged off the recurrent suggestion that his crisis over the agreement might be driven by political ambition and personal calculation. Whatever course he chose, the 35-year-old MP betrayed acute awareness only of the traps and snares, consequences and downsides. He might have been the centre of attention - the man in the eye of the storm - but he plainly wasn't enjoying it.
The pressure was immense and growing. A telephone call from the Prime Minister was followed by a series in quick succession from prime ministerial aides. They finally produced a result - but not the one for which Mr Blair had fervently hoped and for which he had devoted such effort.
Mr Donaldson's instinct on Thursday night had apparently been to seek further clarification from Mr Blair about the legislation establishing the Northern Ireland assembly and providing for the planned programme of prisoner releases. But after consultation with his closest colleagues late yesterday morning, he finally concluded that the end result would be the same.
Mr Donaldson must have known all along that the Prime Minister could not unilaterally alter the multi-party agreement. That he persisted as long as he did - much to the consternation and private contempt of anti-agreement campaigners - certainly persuaded British ministers that he was genuinely torn. Yesterday he decided that the moment of truth had arrived. He maintained the "ambiguities" about the conditions for Sinn Fein's membership of the executive, and the actual "linkage" between this and prisoner releases and the issue of decommissioning paramilitary weapons remained; and in consequence said he still could not "recommend" the agreement to the people.
The Ulster Unionist leadership had resolved to shrug-off the confirmation of Mr Donaldson's defection. Having anticipated just such an outcome, Glengall Street was ready to argue that each of Mr Donaldson's difficulties - outlined on paper to Mr Blair last week - had been addressed by the Prime Minister's Balmoral speech on Thursday night. Party sources were also hoping that the damage to their position would be offset by damage to Mr Donaldson from a "wimp factor" they say has characterised his position ever since Good Friday.
But there could be no denying the damage. Curiously, had Mr Donaldson opted for a No straight-off, then both he and Mr Trimble might each have found the past few weeks slightly more comfortable. But by virtue of the tactical position he adopted then, Mr Donaldson had become - to some degree at least - the litmus test for those unionists wanting to approve the agreement but unpersuaded that they could or should.
On Thursday night Mr Blair sought to assuage the genuine fears of many unionists about the terms on which Sinn Fein would gain entry to a new Northern Ireland Executive, and paramilitary prisoners early release. But it is no exaggeration to say he was also speaking directly to Mr Donaldson. And, as surely as his return to the fold would have been good news for Mr Trimble, so his decision to reject Mr Blair's overtures and assurances is a significant boost for the unionist No campaign on the final weekend before the poll.
The obvious and immediate significance lies in the context of yesterday's Irish Times poll showing a majority of unionist voters intent on voting against the agreement. Despite Mr Trimble's continuing personal confidence, the leaders of the No campaign were last night more convinced than ever that the momentum is with them. And they understand, as well as Mr Trimble, that clear evidence of a unionist majority voting No next Thursday would have a powerful impact in setting the context for the crunch assembly contest.
Depending on how he chooses to play his hand now, Mr Donaldson, too, could have a major influence in shaping the context for an electoral battle meant to confirm Mr Trimble's ability to work the agreement through the proposed assembly.
On past form, Mr Donaldson seems likely to carry his fight within the UUP rather than join other dissident colleagues on the united unionist platform alongside Mr McCartney and the Rev Ian Paisley.
He says he has not decided whether to contest the assembly election. And he plainly wants to continue his disagreement with Mr Trimble in the gentlemanly terms they have maintained since Good Friday. Moreover, he is holding out the possibility that he and Mr Trimble could yet realign around a policy position in relation to the assembly, and Sinn Fein's role within it, which would enable him to support Mr Trimble in the assembly election while still voting No next week.
But the emerging Donaldson stratagem for a restoration of UUP unity almost certainly spells a crisis-in-the-making for the agreement itself.
Prior to last night's announcement, Mr Donaldson told The Irish Times: "David Trimble has indicated to me that the party would not sit in a shadow executive with Sinn Fein in the absence of a clear declaration by the IRA that their campaign of violence is over and that there has been actual and is ongoing decommissioning. If that's the party's position - and obviously that would follow through to the executive whenever the powers are transferred - then that is a position I can and will support in terms of the specific issue of Sinn Fein holding ministerial positions."
At a press conference yesterday morning, Mr Trimble was again casting doubt on the necessity for the creation of the executive in "shadow" form on the heels of the assembly election, saying: "There will be no shadow executive until we decide there is going to be one."
At this writing it is unclear what attitude the SDLP and Sinn Fein would take to any delay, although there is an impression from government sources of some potential flexibility in terms of timetabling, given the time required to pass the substantive legislation effecting the agreement, and that it is not anyway envisaged that powers would be transferred to the assembly until next January or February.
However, what is absolutely clear is that neither party, nor the Irish Government, would countenance any attempt to re-introduce the decommissioning requirement as a pre-requisite for Sinn Fein's participation at ministerial level.
And if Mr Donaldson persists in seeking to bring the Ulster Unionist Party to that position, then talk of unity stratagems can be forgotten: he and Mr Trimble are locked on a collision course which, whatever Mr Donaldson's desires, puts him logically and inevitably on the side of those seeking an anti-agreement unionist bloc in the Assembly.
Whether or not Mr Donaldson contests the assembly election is not of crucial significance. Certainly some of those to whom he listens might be expected to encourage him to remain above the fray - calculating, as one close to him put it recently, that he has positioned himself perfectly for the day "when the wheels fall off this thing".
Whatever his protestations, he has to be hoping and calculating that they do. If they do not, and Mr Trimble wins, Mr Donaldson will not be lightly or swiftly forgiven.