What went wrong with David Trimble's campaign? It is easy to pick up on subordinate themes. Now that Jeffrey Donaldson has hardened his position, it may very well be that the only way to conciliate Mr Donaldson involves Mr Trimble breaking his alliance with Tony Blair, and everyone - outside Lagan Valley at least - can see the relative importance of the two politicians to Mr Trimble.

But matters were not quite so stark three weeks ago. If Mr Donaldson had been allowed to stand, the total Ulster Unionist vote would have been significantly higher and, at worst, a rather soft No if Mr Donaldson had been elected as against a hard No in the shape of the UKUP's Paddy Roche.

The Ulster Unionist manifesto assumed that the electorate was keen to talk about the economic and social issues which the Assembly will tackle. In fact, the electorate was still obsessed by broad constitutional themes.

Mr Trimble should probably have kept the focus more on the agreement and his areas of success, such as securing nationalist recognition of the legitimacy of Northern Ireland's place within the UK. He may also have left his impressive elaboration of his vision of a post-Orange civil unionism a little too late.

Yet almost every vote at every level within the UUP, and almost every poll of UUP supporters, has told us the same story for weeks. Most Ulster Unionists, certainly over 70 per cent, support Mr Trimble's new approach, but a significant minority do not.

Either Mr Trimble recruited more support from previous non-voters, as he did in the referendum but was unable to do on a reduced turnout last week, and/or he stole votes from the Alliance Party, which has a remarkably resilient core vote.

In the end, he did neither, and only Mr Trimble's amazing run of successes should have blinded us to the fact that this was always likely to be the case. The balance with unionism (that is large-U unionism, excluding Alliance) is perceived to have shifted from one slightly in favour of Yes to one slightly in favour of No.

It is a shock to the system. However, there is no question mark against the democratic legitimacy of the agreement, and enough pro-agreement politicians have been elected to guarantee the smooth working of the proposed institutions.

The Rev Ian Paisley seems to think the agreement can be stopped in its tracks, but his more astute DUP lieutenants are more cautious and thoughtful about the future. Peter Robinson will have registered the big pro-agreement vote in his constituency.

For Mr Trimble and his supporters there can be no going back, and before his likely election as First Minister he still holds some important cards.

The majority of unionists will be dismayed by this week's display of intense division. The vast majority of the unionist electorate wants the Assembly to work.

Above all, there is plenty to encourage those in the Ulster Unionist Party who want to work closely with the SDLP in the Assembly. As Seamus Mallon has put it: "Given the underpinning of the principle of consent, the scope for involvement by the unionist community is much greater in terms of being able to create a settlement which is not jaundiced by the past and which is not damned by the future."

As Mr Mallon has added, with considerable insight: "The reluctance in unionism at the present is this absolute fear and suspicion that anything that is agreed or set up is actually subterfuge, a united Ireland by the back door."

The rhetoric of the SDLP in recent weeks has been instructive. It has celebrated the agreement as the final death blow to crude majoritarianism in the North. It has hailed the new recognition of diversity. But it has not presented the agreement as inherently transitional to Irish unity.

It is often said that the agreement represents SDLP thinking more than that of any other party. In a broad sense, this is true, but it also hides some important realities.

In the late 1980s and early 1990s, the SDLP flirted with concepts of "joint authority", even that Britain should be a persuader for Irish unity. Both notions were absolutely unacceptable to those who are now pro-agreement unionists. The agreement represents a step back by the SDLP or, more accurately, a return to the more generous and idealistic themes enunciated by John Hume in the 1970s. (If the agreement represents anyone's game plan in its totality it is the concepts long prevalent in the NIO).

But the SDLP's embrace of the theme of co-operation, the dominant note of its campaign, has allowed Ulster Unionists to respond, at long last, with the necessary warmth.

This provides the context in which the two parties can work together on a consensual basis. Mr Trimble's relative electoral setback, which he has frankly acknowledged, becomes, in a perverse way, a kind of strength.

At a minimum, both governments will have to respect the letter, and spirit, of the deal on practical cross-Borderism they made with him during the last week of the talks. This now looks to be one of the trickiest issues facing the Assembly. Mr Trimble will want to return the largest possible measure of unionist support for the final cross-Border package, and it is in everyone's interests that he is able to do so.

Finally, it is worth noting the greatest irony of the election. Unionist division has helped the republican movement complete its acceptance of "Sunningdale Mark 2". That movement now appears to have found its "honourable way out".

The unionist community, quite understandably, does not accept this. It fears that the war is not over. If it becomes apparent that it is, this more than anything will strengthen Mr Trimble's position. So, of course, would a little help from the Irish Government over Drumcree, as is suggested in some weekend reports. Let us be clear about this - it is amazing that we have come so far. It is now clear that the referendum did not signal the "end of history" in Northern Ireland. Even on that day, the Irish Times/ RTE exit poll contained the warning signal: the Ulster Unionist support base at 19 per cent was lower than the DUP/UKUP combined at 20 per cent.

Mr Trimble will be reluctant to govern Northern Ireland in alliance with nationalists if it means defying the wishes of a majority of unionist Assembly members, and on paper at the moment he has the support of only a thin majority.

However, the longer the agreement lasts, the more likelihood there is of the DUP playing a role in the administration and removing a barrier to an inclusive settlement.

It is going to be real white-knuckle stuff over the next few weeks, but the odds still favour the view that if the agreement is implemented in all its key provisions then a broadly-based "historic compromise" will have been achieved.

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