The stakes could hardly be higher. Mr David Trimble needs to win today, and win big. Opinion polls are all very well. This week's, in this newspaper, provided a tremendous fillip to the pro-agreement forces. But as the dissident MP William Thompson maintained, it's what happens in the polling booth that counts.

And when the votes of the members of the Ulster Unionist Council are counted this afternoon, Mr Trimble will be hoping to win by a decisive margin, knowing that the scale of his victory (or the narrowness of it) will have a crucial impact on the psychology of his party as it prepares for the electoral challenges ahead.

He will need to show courage and candour in equal measure. The courage we can take for granted. Whatever his ultimate fate, the all-inclusive, all-embracing nature of last week's British-Irish agreement bore the imprimatur of a man prepared to take quite extraordinary political risks. Almost as soon as he acquired the party leadership, David Trimble signalled his lack of appetite for the longevity which had marked James Molyneaux's tenure. Those who know him confirm his sense of a life beyond politics. But it will have to wait. There is little more impressive than the spectacle of a political leader battling for survival. Having committed himself, Mr Trimble may be expected to fight like fury.

The public demeanour this past week has been studied, calm, statesmanlike. Behind the closed doors of the Europa Hotel, it will be a case of jacket off, sleeves rolled up, direct engagement and very straight talking - much, perhaps, in the style associated with Tony Blair in his dealings with a Labour Party resistant to change.

Mr Trimble is not, however, in Mr Blair's happy position. Formidable forces are ranged against him. It has always been foolish for the media to dismiss Dr Paisley and Mr Robert McCartney because they weren't party to the talks process. Between them they mustered 43 per cent of the unionist vote in the Forum elections. Dr Paisley still retains his vote-winning potential, and Mr McCartney demonstrably pulls at the heartstrings of many members of the UUP.

In September 1995 the UUC overlooked Messrs William Ross, Martin Smyth, Ken Maginnis and John Taylor in favour of David Trimble. But he was then emergent as the hero of Drumcree, elected by a party disillusioned with the final Molyneaux years and leaning significantly to the right.

Unsurprisingly, it was some of Mr Trimble's "young Turks" who took fright last Friday as the scale of the new deal became apparent. And even if, as one academic ventured the other day, some of Mr Trimble's internal opponents suffer a charisma deficiency, it must be allowed that they reflect very real and deep-seated unionist instincts. It must also be allowed that they are neither mad nor bad because they find many aspects of the proposed settlement extremely unpalatable.

Mr Trimble knows that many of his opponents are totally irreconcilable, that calls from some for clarification is code for conditions which simply cannot be met. Yet he undoubtedly shares the concerns of others in his ranks about the proposals on policing, and prisoners, and above all on the prospect of Sinn Fein entering a Northern Ireland government while the IRA maintains its weaponry intact.

This represents the area of greatest danger to Mr Trimble. It is also the issue demanding the candour on which all could turn.

Mr Jeffrey Donaldson MP has positioned himself very neatly between the leadership and its opponents, suggesting that he could live with the agreement if Mr Blair's "commitments" on decommissioning weapons were enshrined in legislation.

This has been taken to mean that the legislation effecting the agreement should make explicit that Sinn Fein's participation in the proposed Executive should be conditional upon prior decommissioning of IRA weapons. It is a clever move, which may well prove attractive to some senior party members keen to rein in the leadership without prompting a wholesale split.

But Mr Trimble and Mr Donaldson must both know it is a proposition capable of unravelling the entire agreement, and therefore incapable of being fudged. It suggests the establishment of an exclusion zone which would certainly enable Sinn Fein to cry foul, and which would run counter to the inclusive deal for which Dublin and the SDLP (as well as Mr Blair) have signed up.

Friends and admirers of Mr Donaldson last night remained divided as to whether he remains a true admirer of Mr Trimble or is actually pitching to lead a united unionist coalition against the agreement. But if he succeeds in reinstating the precondition for Sinn Fein's full participation in the proposed new structures, Mr Trimble will be the loser.

The potency of the threat is in the simplicity of the argument, its rootedness in the long-standing unionist demand on decommissioning, and its stark appeal to those who find the thought of Mr Adams in government simply unthinkable.

Unless he is to be boxed in to a commitment he must know he cannot deliver, Mr Trimble will have to persuade his party that the text of the agreement - and the simple necessities of entering into the world of democratic government - will compel Sinn Fein to commit itself to "the total disarmament of all paramilitary organisations" and "the decommissioning of all paramilitary arms within two years" of endorsement of the agreement.

He can also be expected to argue forcefully that decommissioning is but one of the challenges facing Sinn Fein if it is to claim its place in arrangements rooted in the principle of consent. Seamus Mallon has candidly acknowledged that the agreement settles the North's constitutional position, and recognises the legitimacy of its place within the United Kingdom.

Partition may eventually be ended by consent of the people, but it is the basis of this proposed settlement for the here and now. And as participants in government, the British and Irish administrations - no less than Mr Trimble - would surely require Sinn Fein to fashion a commitment to peaceful and democratic means which would, among other things, be reflected in its attitude to republican dissidents who might seek to destroy the new arrangements by continuation of the war.

It now seems clear that somewhere along the road of an initially strategic engagement Mr Trimble came to accept that the Sinn Fein leadership might be serious about bringing the war to an end. It is arguable that he should have done more to prepare his party, although with hindsight that may not have been possible. The consequence, however, is a party shell-shocked by the scale of change it is now asked to embrace.

And from Mr Trimble's perspective, it must embrace it in an emphatic manner. Some outsiders yesterday appeared surprised by the UUP leader's expectation of more than a 70 per cent endorsement, thinking a smaller margin would see him set fair. Mr Trimble knows better.

Victory on a large scale would give his party the confidence that it had made up its mind, and impinge directly on the constituency associations as they begin the task of selecting candidates for the Assembly elections. A close-run thing would invoke the memories of 1974 and the spectre of "pledged" and "unpledged" unionist candidates battling each other in the elections for an Assembly in which the combined anti-agreement bloc could emerge the largest.

We can be sure that spectre will attend today's meeting of the UUC. We can be sure, too, Mr Trimble will oblige his delegates to ponder what Mr Blair's response might be if confronted with such a scenario.