The issue of decommissioning is the Sword of Damocles of the peace process. It undermined the first IRA ceasefire and, although the second republican cessation looks secure for the present, the "D" word could wreck plans to set up the North-South Ministerial Council.
As an inevitable consequence, the Assembly itself would be aborted. Most observers believe the shooting would not necessarily start again - at least, not for some time - but the entire edifice so carefully constructed in the weeks and months leading up to Good Friday last would crumble and fall.
This is a thought to horrify many people, but not the hardline elements of unionism which are deeply suspicious of the whole project, which they see as part of a softening-up process leading to a United Ireland.
The real test of strength for the different factions of unionism is expected to come on October 24th, when the UUP meets for its annual conference in Derry. Rightly or wrongly, that is seen as the day when the fudging has to stop.
Observers say Mr Trimble either has to stand up to his hardliners and secure the open and public acclaim of his party for a pragmatic approach, or else decide that "If you can't beat 'em, join 'em."
Until now he has always managed to come out on what peace process supporters would regard as the side of the angels. At the very beginning of the talks in 1996, he came under terrible pressure to oppose the nomination of Mr George Mitchell as chairman, but ultimately held firm.
As for Good Friday, who can ever forget how the UUP leader held out against the No wing of the party? According to insiders, he told his party delegation, "I am doing it," and then left the room.
One long-time "peace processor" on the nationalist side calls decommissioning a "virus". Once again, Mr Trimble has to cope with this virus.
His public and official position is that there must be decommissioning before Sinn Fein can join a shadow executive, and this does not seem to leave him much in the way of "wiggle room". But in the past he has somehow or other always managed to find a way out of the impasse, whereby decommissioning remained an issue but the peace process also stayed intact.
This time around it seems even more tricky than in the past, and even David Trimble may finally have run out of options.
Furious work has been going on behind the scenes for months now to resolve the problem. As on other issues, such as the Drumcree standoff, Dublin looked initially for a point midway between the two opposites.
Suppose the IRA were not to decommission now but to give a timetable for when it might do so in the future? The republican response was along the lines, "Which part of `No' do you not understand?" Early indications that Mr Trimble might "buy into" a timetable also proved illusory.
Dublin now seems to have come around to the view that the "softly-softly" approach is best, both in terms of securing the ultimate destruction of weapons as well as the more immediate aim of ensuring the survival of the peace process.
Moves are afoot but nothing will be done in public before October 24th, insiders say. The strategy is not being disclosed but is believed to involve "sounding out" the parties on both sides, i.e., republicans and unionists, about different possibilities.
Insiders retreat into vagueness when asked what precisely is going on. But certain basic elements of the situation are discernible.
Firstly, there will be no decommissioning in the immediate future. Secondly, the IRA may not be prepared to consider decommissioning in practice but would probably be willing to discuss the idea in theory. Thirdly, when the republican movement feels it no longer needs to hold on to weapons, then it will decommission them.
General John de Chastelain's role will, of course, be crucial. He is being put forward as the guarantor of good faith, just as the three-member decommissioning body was in the past.
Republicans have convinced Dublin and the more percipient elements in London that there will be no hardware forthcoming but that there might be what diplomats like to call "language".
Indeed, we might even see a rerun of the "soft cop, hard cop" routine that republicans performed on the Mitchell Principles:
Sinn Fein accepted them but an IRA spokesman said there were difficulties.
The UUP leadership may find this very cold comfort. Republicans will snort that, "Trimble painted himself into a corner and it's not up to us to dig him out".
But the question unionists must face is whether to hold out for "product" and thereby lose the Assembly, the changes in Articles Two and Three (which are conditional on the rest of the Agreement being implemented), and possibly in the longer term the ceasefire itself.
There are signs that the "silent majority" of unionism - which bestirred itself to vote for the agreement on May 22nd but did not brave the rain to give Mr Trimble a thumping majority in the subsequent elections - may be coming back to life. Editorials in two Belfast newspapers recently pointed out the stark consequence of an unduly fervent stance on decommissioning.
Mr John Hume has pointed out repeatedly that if the paramilitaries decommission on Monday, they can "re-commission" on Tuesday. The two governments - especially Dublin - are loath to let the October 31st deadline for the formation of North-South bodies slip away. A few days will not matter but serious slippage will be a demoralising and potentially fatal blow.
The last week of October promises to be a nerve-wracking time for negotiators and mandarins. Any deal is likely to be done at the last minute with promises, arm-twisting and crisis meetings.
It will, as one hardened negotiator put it, "be like deja vu all over again."