Over the last few days much has been said and written about the likely result of the Ulster Unionist Council vote on Saturday. Figures are quoted liberally by pro- and anti-deal factions.

While it is probably fair to say that one-third of delegates are firmly pro-Trimble and another third are firmly against, the truth is that no one can predict the result accurately.

I have some experience of misreading the council. In October 1995 I ran John Taylor's campaign for the leadership. Very quickly it was firmly established in the media psyche that Taylor was clear favourite to win. Even David Trimble's late entry into the race didn't upset the media consensus.

Usefully, Taylor was endorsed by the Daily Telegraph and the London Times, as well as the Belfast Telegraph and the News Letter. Of more dubious value, the nationalist Irish News came on board, a fact used to good effect by David Trimble in his selection address.

The Taylor campaign was not all spin. Journalists in conversation with unionist opinion-formers, local party grandees and district councillors were left with the same impression: the Ulster Hall would be a coronation. When the first set of ballots was collected (in orange buckets), the party hierarchy on the stage were visibly aghast.

Shudders of horror went through the comfortable British-Irish Association gathering at Emmanuel College, Cambridge, and many had to retire to the bar in need of a stiff drink. Trimble's astonishing success proved how resistant the unionist grassroots are to spin. The only two men not in the least surprised were Taylor and Trimble.

What went wrong from Taylor's point of view that night? A poorly delivered speech certainly did not help but, with hindsight, was not crucial. The truth is that although most of "middle Ulster" stayed loyal to Taylor, the descamisados were Trimbleista. The vast majority of those people will not be canvassed by the local press corps for their views of the deal currently on the table.

The influence of Drumcree in determining Trimble's success has often been overestimated. Dancing through the streets of Portadown with Ian Paisley is a dubious advantage in a party which is defined largely, perhaps mainly, by its opposition to the political and religious zealotry of Paisleyism.

Trimble and Taylor both stood on a modernising platform, anxious to promote the unionist case more vigorously in Britain, the US and Europe. While Trimble had less experience in elected politics - and, therefore, less opportunity to make enemies on the way - the crucial difference between their two pitches to the selectorate was political.

Taylor had embarked on a highly successful tour of the Republic explaining unionist opposition to the Framework Documents while at the same time promoting a plan for a new devolved administration for Northern Ireland incorporating all the main parties, including a suitably cleaned-up Sinn Fein.

His injuncture for North and South to "reason together" while bringing in the paramilitaries - the critical missing element at Sunningdale in 1973 - was an idea before its time. Many ordinary unionists held a principled objection to Sinn Fein in government, however reformed.

In the space of three years, Trimble reversed that opposition and won 70 per cent backing from the UUC for an involuntary four-party coalition. The question of the republican movement's commitment to democracy and non-violence was left unresolved.

The failure of the IRA to decommission alongside talks as Senator Mitchell had proposed, the continued mutilations, the occasional murder and the disappointing Assembly election result made the achievement of disarmament a political necessity for Trimble.

All talk of the republican movement being able to establish its peaceful credentials by "equally effective means", as had previously been mooted, had to be dropped. The fact that the agreement envisaged Sinn Fein sitting in a devolved executive with substantial autonomy further necessitated a tangible symbol of changed hearts.

The intransigence of the IRA on this point was felt at the last meeting of the UUC in March 1999 when a significantly anti-agreement party officer team was returned, pro-agreement candidates polling poorly. The combined number of votes for anti-agreement candidates outstripped that for Trimble loyalists.

This all goes to demonstrate the highly unpredictable nature of the meeting just three days away. Trimble certainly has the media and civic society behind him but, as was shown in 1995, that will not, in itself, be sufficient.

Most unionists want to trust Trimble's instinct, but he will need to demonstrate to a sceptical audience completely unversed in the nuances of Provo-speak that the statements by the IRA and Sinn Fein last week amount to an end to conflict and a reversal of the previous non-acceptance of the principle of consent which underpinned the armed struggle.

He has a powerful case, helped in no small part by the ghastly prospect of explaining away a rejection to British and Northern nationalist opinion of a repudiation of the deal. Then there is the likely effect on community and business confidence of seeing a Nobel Peace Prize-winning First Minister forced out.

Some seem convinced that the republican movement has accepted that decommissioning is the inevitable logic of the current process. Others have never accepted powersharing: Powellism still stalks unionism's Glengall Street headquarters.

The middle ground suspects a republican ruse to enter government without any real intention of decommissioning. They will need concrete assurances about the consequences of the IRA defaulting and when they will come into play. Mo Mowlam was not the person to persuade them. Peter Mandelson might be. Worryingly, a poll of the UUP conference last month found that only a tiny minority still trust their own Prime Minister after the handwriting fiasco.

As I have found myself, it is a mistake to take the Ulster Unionist Council for granted, even when the tide does seem to be running strongly in one direction.

Stephen King is a special adviser to John Taylor. This article is written in a personal capacity