This is a time of great danger for the Belfast Agreement. Some in "official" Dublin (though not all) appear to be in a state of denial about the deadly potential of this week's South Antrim by-election, and the nationalist standoff over the British government's implementation of the Patten policing reforms.

Yet the two, separately or together, could break David Trimble's tenuous hold on Ulster Unionism. One senior Irish Government source recently confided his difficulty in accepting that upon the outcome of a single by-election could hang the fate of an international treaty, endorsed in separate referendums by the people of Ireland North and South. By-elections are notorious vehicles for protest, he reasoned, as the Government itself was reminded recently. Yes, he understood defeat for Mr David Burnside could cause problems for Mr Trimble, but whatever the result, he affirmed, the two governments would remain committed to the agreement.

Well, bully for the two governments. The point is that South Antrim is no ordinary by-election, which a governing party might expect to lose, and regain come a general election. The essential context is the seepage in Mr Trimble's support, from a 71 per cent high in April 1998 to the narrowest of victories - 53 per cent to 47 per cent - at the last meeting of the Ulster Unionist Council.

An analogy might be drawn with the South Belfast by-election in 1982, caused by the murder of the Rev Robert Bradford. The DUP had inched ahead of the UUP for the first time in the preceding local government contest, and the fear in Glengall Street was that the loss of South Belfast would see the DUP emerge as the party of majority unionism.

Against the backdrop of two "worst ever" shares of the unionist vote - in the Assembly and European elections - defeat in South Antrim would spell the writing on the wall for many sitting Ulster Unionist MPs. A fresh leadership challenge would almost inevitably follow. When they opposed Mr Peter Mandelson's suspension of the Executive in February, Dublin and the SDLP appeared almost sanguine about the possible loss of Mr Trimble's leadership. In Mr Mallon's latest positioning on the Patten report - it should not be sacrificed to keep any political party on-side - there is, again, more than the hint that Mr Trimble should not be considered indispensable.

This strikes British ministers and officials as denial on an extraordinary scale. For where is the agreement if Mr Trimble loses the leadership of his party? Faced with the internal unionist logic, leading nationalists fall back on the notion that it would be "up to the unionists" to find an electable replacement First Minister. Yet the existing Assembly arithmetic says this would be impossible. That is why it was reckless for Dr Mo Mowlam's Northern Ireland Office to have implied Mr Trimble might follow suit after Mr Mallon's resignation as Deputy First Minister designate last year. Even then, Mr Trimble didn't have the votes to ensure re-election, a fact belatedly confirmed when Mr Mallon, against his wishes, had to withdraw his resignation. Moreover, the Ulster Unionist Council would hardly depose Mr Trimble now in order to replace him with another, and by definition, less able, pro-agreement champion.

There would, in that event, for sure, be some Trimbleistas who would choose the Faulkner-UPNI route and seek to carry on - but not enough. All reliable indicators suggest at least half the Assembly members would bow to a democratic decision of the UUC, and thus give the anti-agreement forces the requisite 60 per cent wrecking share of the unionist bloc at Stormont.

A DUP victory in South Antrim, therefore, could trigger an internal Ulster Unionist revolt that would end in the withdrawal of the majority community's consent for the Good Friday accord.

Which is not to say the agreement, or Mr Trimble, is safe if Mr Burnside wins. Much would turn on how Mr Burnside interpreted any margin of victory. Though the DUP obviously makes the charge, Mr Burnside is certainly not a sceptic just for the duration of the by-election campaign. His growing disillusion with the way the agreement has been implemented mirrors the experience inside the UUC, and in the wider unionist community.

Between now and Thursday he will be prioritising the message that he, and not the Rev William McCrea, is the better protest vehicle for No voters.

At the conclusion of the referendum count, when, it should be remembered, almost half the unionist population said No, Mr Tony Blair seemed to promise the unreconciled they would not be isolated or ignored. We can be certain they feel they most certainly have been. We may be certain, equally, that a successful Mr Burnside would not so lightly forget them.

To the contrary, a freshly elected Mr Burnside could be expected to bring the full weight of that unionist alienation to bear in all his dealings with Mr Trimble, and in all his party's dealings with the British and Irish governments, the SDLP and Sinn Fein.

Not for David Burnside MP the lot of the newly elected member, left to quietly learn the ropes at Westminster. Election on Thursday would make him instantly a major player at the heart of his party's decision-making process.

Nowhere is he likely to bring his influence to bear more swiftly than in the ongoing debate about Patten and the future of policing. With Mr John Taylor facing a serious deselection threat in his own constituency, these two together are certain to resist the pressure for further significant concessions to nationalist and republican opinion.

The unionist leadership knows that Mr Mandelson's concession - keeping the name of the Royal Ulster Constabulary in the so-called title deeds of the Police Bill - is but a figleaf, and Mr Trimble is as incensed as any of his colleagues at the SDLP's apparent determination to strip even that away. One unionist minister has warned privately that it is the policing issue, and not decommissioning, that might yet break his leader.

Mr Mandelson knows all this. His fear is that, even if Dublin and the SDLP understand the unionist realities, they might yet disregard them, and with potentially fatal consequences for the agreement. For the Secretary of State does not share Mr Mallon's view that Mr Trimble is dispensable. Nor does he see a choice between Patten and the agreement. Mr Mallon correctly counters that the agreement could be fatally undermined anyway if the Catholic community is not persuaded that the finished product represents a genuine beginning for policing in Northern Ireland. Nor should anyone doubt the pressures on Mr Mallon - for it will be down to him - as he prepares to make a truly historic judgment call.

In his latest attack on the Secretary of State, Mr Mallon has set the stage for a colossal test of wills. And the only certainty for Mr Mandelson is that the task of reconciling Mr Mallon and Mr Trimble will be greater, whatever the outcome in South Antrim.