After the week-long battle of spin, the reality: David Trimble is facing the fight of his political life. Last week's bullish predictions of an overwhelming majority have given way to the cold appraisal that victory is not assured, and defeat certainly not impossible.

Everything - his leadership, the future of unionism and the survival of the Belfast Agreement - will almost certainly turn on Mr Trimble's performance before the assembled ranks of the Ulster Unionist Council in the Waterfront Hall this morning.

There is no scientific evidence, no way of knowing how the 858 delegates will finally cast their secret ballots. But at this writing the Ulster Unionist leader is, at the very least, riding for a scare.

Sources close to Mr Trimble say he would be mightily relieved to win by a 6040 margin. Their real fear, however, is that support for the devolution/decommissioning deal could dip significantly below the 60 mark, ensuring nothing but a succession of tilts against the leadership all the way to the annual meeting of the party in March.

On the most benign assumption, of course, any majority will do. In this golden scenario Mr Trimble's doubting Assembly members accept the majority verdict; the executive is successfully launched; and, before the rejectionists can credibly stage a rerun of today's epic battle, the Provisional IRA will have started the process of decommissioning its weapons.

The UUC delegates know the theory. Many of them, supporters of Mr Trimble included, think it a poor thing, certainly in comparison with what they have been led to expect these past 16 months. They know the judgment Mr Trimble has made about the bona fides of Gerry Adams and Martin McGuinness.

But their final judgment - for Mr Trimble spelling the difference between short-term survival and the commanding lead necessary - will more probably rest on what he can tell his party will happen if his trust proves to have been misplaced and the IRA fails to deliver.

None can doubt it: David Trimble has taken, is taking, a hell of a gamble. The scale of it has become steadily more apparent with each passing day from the conclusion of the Mitchell review to this morning's historic gathering of unionism's ruling body.

In words presumably intended to spell reassurance, Mr Adams declares: "There is no secret deal." Mr Trimble's doubters are only too ready to believe him. In his private consultations with colleagues, confirmed publicly by Jeffrey Donaldson, Mr Trimble certainly has been making no claim to the contrary.

There is no private understanding; no actual promise; no firm date for the actual start of IRA decommissioning. For all that he proceeds on the working assumption of republican "commitment", there is nothing Mr Trimble can point to in the Mitchell deal which provides absolute "clarity of intention" or "certainty of achievement".

Mr Trimble insists that "no guns, no government" remains the principled basis of his policy and intent. He argues that the shift to "government and then guns" is purely tactical and cannot be described as a colossal U-turn.

Above all, he rejects suggestions that he is about to breach the previously-stated principle that unionists would not sit in government with the representatives of a fully-armed terrorist organisation.

What Mr Trimble cannot deny is that the Mitchell sequencing does not, at any rate yet, define the beginning of "a credible and verifiable process of decommissioning leading to total disarmament by May 2000".

So has he, as one British commentator put it, "bought an armed pig in a poke"? Has he, as Dr Paisley claims, drawn another famous "line in the sand" only to see it washed away?

Maybe. Some of his erstwhile supporters in the Assembly party now suspect Mr Trimble might never have been totally serious about decommissioning, that the tortuous twists and turns of the past 16 months reflected little more than the constraints imposed by the outcome of the Assembly election.

AND IF Mr Trimble's dispositions are rooted in what Sinn Fein and the IRA have actually committed to thus far, then many UUC delegates who backed him in April last year may indeed conclude he has "sold the pass." If.

Since the conclusion of the Mitchell review, the widespread assumption has been that Mr Trimble after all those weeks of looking into the whites of their eyes elected to trust Mr Adams and Mr McGuinness. However, that assumption is almost certainly wrong.

Mr Trimble has more probably elected to trust in the process, a process he calculates will either force the IRA to disarm or leave Sinn Fein alone to face national and international opprobrium if they do not.

The temptation here will be to regard this as a variation on that old familiar, the blame game. That, too, would be mistaken.

For Mr Trimble is entitled to say he is planning for success, not failure. If he wins by a sufficient margin today, by Monday night he will head Northern Ireland's first government for a quarter of a century, with ministers McGuinness and Bairbre de Brun seated alongside him at the cabinet table. If it ever was the case, Mr Trimble patently is not now working to the politics of exclusion.

At spectacular risk to himself, and to the party he leads, David Trimble is guaranteeing Sinn Fein's place in the sun. But what, Mr Donaldson demands, have they guaranteed in return?

Steven King (who combines liberal instincts with a sharp grasp of unionist realities) may have done himself no good with Mr Trimble's coterie of aides and advisers. But there was sense to his exhortation: "Tell the truth, David."

The truth is that nothing is guaranteed. Mr Trimble calculates that the pressure will grow remorselessly on the republicans once Sinn Fein is installed in government: that Gen de Chastelain will "put it up to them" with such vigour that, by the end of January, say, the public expectation will be such that the Provos simply would not dare disappoint.

But, comes the insistent demand, what if they do? What if they still think to have it both ways? What if the Ulster Unionist Party has jumped first, only to discover it has jumped alone?

In his interview in yesterday's Irish Times, Mr Trimble showed himself keenly aware of the question to be addressed. He also gave the clearest indication of the answer taking shape in his own mind.

The process, he insisted, was not open-ended. And he would tell his party to trust themselves: "This cannot work without the participation of the Ulster Unionist Party. At the end of the day, whatever anyone else might do or say or think, it is our decision . . . We have within ourselves the capacity to put a term to things."

Mr Trimble said he was against publicly setting a date on which he and his ministers would quit the executive, should decommissioning fail to happen. Despite this, some insiders were agreed that if those words meant anything, they pointed to post-dated letters of resignation. However, with the wording of the motion to be put to the council still not decided, another possibility emerged last night: that Mr Trimble might invite the party to set its own "term".

Mr Trimble may well have concluded that nothing less will entice his deputy, Mr John Taylor, back onside. More crucially still, as he weighs the possibilities of victory and defeat, he may have concluded this is the only means by which he can provide that "certainty of achievement" he has so long demanded of the republican movement but which his party today will be seeking from him.