David McKittrick: From nine MPs, to six, to three, UUP's waning fortunes have left Trimble looking vulnerable

A fomer press officer for David Trimble used to do a passable imitation of a panicking Bugs Bunny, exclaiming in a high-pitched voice: "Oh no! Twimble's in twouble!"

The Ulster Unionist leader has spent so much of his eight years at the top in trouble of one sort or another that his leadership has been one of lurching from crisis to crisis, with his position often in doubt.

He has survived so many crises that he is routinely regarded as the man who never really prevails decisively within his party, but who is never really defeated either. Yesterday, the buzz was that this crisis might be fatal, and this time he faces a real fight for his political life.

The fact that the Ulster Unionist Party is divided is beyond dispute, and has been for decades. It has long been the most untidy and unruly of Northern Ireland parties, with three main internal cleavages. One section, led by Mr Trimble, believes in accommodation and modernisation and reaching a new deal by negotiation. Another, epitomised in his predecessor, Lord Molyneaux, consists of hardliners set against a deal. The third section wobbles between the two, sometimes supporting a new start and sometimes retreating to the laager.

In practical terms this has generally meant that in recent years Mr Trimble has received between 50 and 60 per cent of votes at recurring Ulster Unionist Council meetings called by his opponents.

This has not been a comfortable position for either the Trimble or Donaldson camps: to extend the cartoon metaphor, one has always been just in front with the other hard on his heels.

Mr Donaldson insists this is not about personal ambition, though it is clear that if the pursuing pack catches up with Mr Trimble he will be a goner. It is also clear enough that Mr Donaldson is the leading contender to succeed him.

The perennial complaint about Mr Trimble is that he has not done enough to sell the advantages of the Good Friday Agreement to the Protestant doubters. Whatever the merits of this criticism, electoral support has been leaching away.

Its voting strength is at its lowest ebb ever, with support for the Rev Ian Paisley's Democratic Unionist Party growing. The grass roots pine for a revival and yearn too for unity, but it is clear that neither Mr Trimble nor Mr Donaldson can deliver the latter. Tony Blair has made it obvious that he believes Mr Trimble is the only Unionist show in town and that he is the only figure capable of delivering enough Protestant support to work the Agreement. This has led to nationalist complaints that the Prime Minister tries too hard to "Save Dave" at the expense of other considerations. But the problem is that Mr Trimble's electoral losses have left him vulnerable to strikes such as yesterday's Donaldson move. The party used to have nine MPs; now it hassix, and three of these have refused to take the whip. Mr Trimble controls three of Northern Ireland's 18 MPs, which is an ominous position for a Unionist party leader.