Are we about to witness "The Trimble Crumble"? The disparaging phrase was coined by a former ally: one of the "young Turks" with whom David Trimble once thought to displace his inherited party establishment, who now spend their waking hours thinking how they might replace him.

It reflects the persistent right-wing critique of the Ulster Unionist leader as a man who, in the words of one adversary, "talks a good fight but folds the minute he comes under pressure". Seldom can an Ulster Unionist leader have known greater pressure as President Clinton rowed in once again behind Mr Blair and Mr Ahern last night; urging him to take one further "risk" for peace; invoking his natural sense of obligation to "our children, and our children's children"; painting for him (should he have any doubt) a lurid picture of the terrible "abyss" into which Northern Ireland could fall should the agreement collapse.

Seldom can an Ulster Unionist leader have known such pressure, in the commanding presence of a prime minister without fear or need of Ulster Unionist support in the House of Commons; convinced that he, on Good Friday last year, had secured Mr Trimble's and Ulster Unionism's most fundamental constitutional objectives, and insistent that failure to move forward now would put that achievement at risk.

And seldom can an Ulster Unionist leader have felt so torn between the instinct (both personal and political) to stay on-side the British prime minister, and the instinct for self-preservation.

One of Mr Trimble's aides was reportedly furious last night to find a local broadcaster speculating on the likelihood of a challenge to Mr Trimble's leadership should he agree to enter government with Sinn Fein without a start made to IRA decommissioning. A threat to the leadership was not an issue, he insisted, because the possibility simply didn't arise: the Ulster Unionists were completely at one behind the declared party line.

But, in the grounds of Castle Buildings earlier yesterday, "at sixes-and-sevens" might more accurately portray the disposition of UUP Assembly members as they joined the endless speculation about the course of the ongoing negotiation inside.

Not all, it must be said, were either fearful or disposed to disloyal or heretical thoughts about the leader. Of course he wanted a deal, and he would stay in the game until the last cards were dealt, said one. But this loyalist was clear that the famous line-in-the-sand had been drawn and would not, this time, be washed away.

Others were far less sure. They could smell something cooking; suspected Mr Trimble's instinct would be to oblige the prime minister; were fearful that he seemed still to be conducting lengthy meetings with Mr Blair and Mr Ahern on his own, and that they weren't being told "the whole story"; and were bracing themselves for the moment when the prime minister himself would confront them for the final, hard sell.

Dark, threatening clouds hung over Stormont for most of the day. And they reflected the mood and temper of some of Mr Trimble's most inflexible colleagues as they prepared themselves for what they assumed would be the worst.

One backbencher told The Irish Times that if his leader emerged with anything short of "simultaneous" decommissioning he would immediately sign the DUP-sponsored motion, ensuring an immediate priority Assembly debate on a motion to exclude Sinn Fein from the executive.

Utterly convincing in his intent, he expressed confidence that his defection alone could be enough to scupper any deal - because the pressure would then be on the other potential dissidents as they faced the appalling prospect of a roll-call vote on whether or not Sinn Fein should be in government.

There would be no question, of course, of carrying the motion to exclude Sinn Fein, thanks to the Assembly's cross-community mechanisms. They key question would be how the internal unionist vote would fall - and, crucially, what that would portend for Mr Trimble's future capacity to command the necessary share of the votes to maintain authority and control in the Assembly.

That authority and control, and the capacity to deliver the requisite 40 per cent of the unionist bloc, would be lost if just five Ulster Unionists quit the Trimble camp. That would produce a situation in which Mr Trimble could count on just 24 Assembly members, while the anti-Agreement bloc would have 34 and, all-importantly, the blocking 60 per cent share of the total unionist representation at Stormont.

Theories persist that in such circumstances Alliance or Women's Coalition Members might change their formal designation to bolster Mr Trimble's numerical position. But the potential rebel was dismissive. In such circumstances, he insisted, Mr Trimble as the leader of unionism would have forfeited all moral authority.

Nor of course would this individual really think to find himself standing alone. He speculated that as many as four others would certainly defect if the deal proved unacceptable. And as journalists confided that they had identified the four or five likely suspects, it soon became clear that there was every likelihood that they could actually number 10 or more.

The more one talked to Ulster Unionists, the more one sensed the all-too-real limits of Mr Trimble's room for manoeuvre. And the more instantly believable, then, when word first emerged from party sources that Mr Trimble had effectively told Mr Blair and Mr Ahern there would be no deal.

At around 3 p.m. Mr Trimble, accompanied by John Taylor, Ken Mag innis, Jeffrey Donaldson, Sir Reg Empey, and Michael McGimpsey, met Mr Blair and told him - apparently emphatically, individually and collectively - that there was no question of an executive being formed without a simultaneous start to IRA decommissioning. They would either "jump together" or not at all.

Much fun attaches to John Taylor's "barge pole" and its unique, if wholly unreliable role, in this process. But there was no playacting hours later when Mr Taylor went on Sky to repeat the mantra. Nor on the 9 o'clock news when Mr Donaldson delivered his turn, again under the watchful eye of Mr Trimble's press secretary.

Mr Adams might insist that the concept is to turn the agreement on its head: but the remorseless unionist message appeared indeed to be "No Guns No Government." To which the obvious retort, from increasingly gloomy SDLP, Sinn Fein and Irish Government sources, was "no government, no decommissioning".

So Mr Trimble faced into the long night, pondering the possible fallout, while seemingly having settled that without a last-gasp surprise offer from Sinn Fein he could not and would not move. And for all their fractiousness, backbench rebels and close allies alike seemed suddenly certain that Mr Trimble would not be surprising his party later.