It ought to be an easy victory for David Trimble today. Look at it from the point of view of an anti-agreement opponent such as Jeffrey Donaldson; there might well be a good moment to take over the leadership of the Ulster Unionist Party, but that moment has not yet come.

At present, a new anti-agreement leadership of unionism would face formidable - in fact crushing - hostility from all sides. Britain and its media; nationalist Ireland, international opinion and a large slice of local unionist opinion - all would say that narrow-minded, unimaginative bigots had destroyed the greatest leader of unionism this century just as he stood on the verge of implementing the most hopeful attempt yet to win a peaceful and prosperous future for Northern Ireland: a vision backed by 71 per cent of the people in the 1998 referendum.

But, in sharp contrast, imagine taking over the leadership of the Ulster Unionists early next year in the event of that republican failure, widely predicted by anti-agreement unionists, to live up to the implicit bargain on decommissioning.

David Trimble could be presented, not as a heroic warrior for peace and the preservation of the Union, but simply as the man who got it wrong. The republican movement could be presented as unworthy partners in any negotiation. The "No" unionists would still not have a credible strategy to resolve the problem - just as they don't bother to make any show of having a strategy today - but it would not matter so much because no one else would have a strategy either.

It is inconceivable that these thoughts have not fluttered across the unconscious mind of anti-agreement unionism and have not somewhat restricted the intensity of its campaign. Anti-agreement unionism has always had the appearance of being defensive; or to put it this way, standing in defence of certain values - democracy, honesty, the Ulster Protestant way - but this is, also, of course, its strength.

The Northern Ireland Office, on automatic pilot and assuming an easy Trimble victory, did not deal with the sore point in the Protestant mind, destined to be reiterated in every anti-agreement speech today. Did not the Prime Minister lie to us in the referendum, so how can we trust any British government pledges today? There is an answer here - one which is rather more credible after the republican statements of last week.

It is to acknowledge that the path to the vindication of the pledges has been unexpectedly slow and tortuous, but at last republicans have broken with the beatings and the threat, not just the actual use, of violence. Soon, if three governments are not to be disappointed, they will begin the task of decommissioning. But it was never said - and that quite simply is not good manners in a society where mode of address matters.

Nevertheless, Mr Trimble has gained much from the arrival of Mr Peter Mandelson: the tone of the discussion of the policing issue has changed in a way that was absolutely necessary. Mr Mandelson has made it clear he is uncomfortable with the motion of appeasement. More substantively, he has signalled twice this week that there will be a return to the status quo of direct rule if the IRA fails to deliver; a status quo that Dr Mo Mowlam once famously declared to be not an option has been restored, albeit reluctantly, as indeed an option. There will be no reward for republican default.

In the summer, eminent Blairistas were astounded by the obsession of the Belfast media and chattering classes with the theme of joint authority, a subject to which the Blairistas had devoted little thought. After all, as Tony Blair declared in his speech in Belfast in May 1997 outlining his political vision for the North; a vision which he has stuck to in every detail: "The principle of consent is and will be at the heart of my government's policies on Northern Ireland.

It is the key principle . . . Any settlement must be negotiated, not imposed." At any time, even the slightest dilution of this principle - in the direction, say, of being persuader for Irish unity - could have bought off the republican movement forever. The decision to hold the line on this point is what allows today's opportunity for Mr Trimble.

In short, unionists can calculate that the IRA will not be rewarded for a failure to decommission while republicans, as a consequence, have every incentive to settle for what is on offer.

But in the background there lies the greatest reason for the delegates to vote Yes today; almost always unstated but deeply significant, the provisional republicans have not been defeated. They have not been marginalised - as so many delegates would have liked - but they can never say, if this agreement is implemented, that what they did during the "Troubles" was right. They cannot say it, because the IRA rejected a very similar deal 25 years ago; and most profoundly of all, because the unionists of Northern Ireland will have shown themselves capable of generosity and realism, and by so doing will destroy the crude stereotyping of an entire people which has justified an unjustifiable sectarian campaign of violence.

Paul Bew is professor of politics at Queen's University, Belfast