Assembly members (MLAs) are expected to vote for a new First and Deputy First Minister next Friday. The positions have been vacant since Mr Trimble resigned in July.
The First and Deputy First Minister, who are elected together, must secure the support of a majority of both unionist MLAs and nationalist MLAs - on joining the Assembly, members designate themselves as Unionist, Nationalist or Other.
In the election for First and Deputy First Minister in July 1998, 30 of a possible 58 unionist votes were cast for Mr Trimble and Mr Mallon. It is a moot point whether they will be again.
If the Ulster Unionist Party executive, as expected, approves Mr Trimble's decision to stand for re-election, he will at least be able to count on the support of anti-agreement colleagues, Mr Jeffrey Donaldson and Mr David Burnside.
Attention is focused, however, on how dissident Ulster Unionists, Mr Peter Weir and Mrs Pauline Armitage, and the two Progressive Unionist Party members, Mr Billy Hutchinson and Mr David Ervine will vote. If Mr Trimble wins just one vote less than in 1998 it could cost him the election.
Mr Weir, who had the party whip removed in January 1999, is the most likely to oppose Mr Trimble. Critical of his party's stance on decommissioning, Mr Weir said yesterday he was still undecided. "What I have heard at the moment is insufficient for me to back anybody for First Minister," he said. Mrs Armitage, who last year resigned the party whip, has not decided her position either. Speaking to The Irish Times yesterday, she seemed unaware of how crucial her support could prove. "I hadn't done the sums, I hadn't realised that my vote was so important," she said.
The Progressive Unionist Party would not say how its members would vote until its Ruling Council had met. A decision is expected by Tuesday.
If any MLAs who voted for Mr Trimble in 1998 withdraw their support this time, his election would depend on the passive support of anti-agreement unionists, or on Other MLAs redesignating themselves as Unionist MLAs.
The 21 DUP MLAs are certain to vote against Mr Trimble. So too is Mr Robert McCartney of the United Kingdom Unionist Party. The three United Unionist Assembly Party MLAs will take their lead from the DUP, according to one, Mr Fraser Agnew.
Of the other anti-agreement groupings the Northern Ireland Unionist Party was thought the most likely to help Mr Trimble by abstaining or being absent on the day of the debate - the First Minister need only win a majority of those unionists who vote. However, party leader Mr Cedric Wilson said yesterday he would not vote for Mr Trimble.
Mr Trimble could win the election comfortably if the seven non-aligned pro-agreement members were to redesignate themselves as unionist, but for this option to work more time would have to be bought for the Assembly.
Thirty calendar days are required for members to redesignate and the Assembly is due to end on Saturday November 3rd if a First and Deputy First Minister have not been elected. There is speculation that before then the Northern Secretary Dr Reid could suspend the Assembly for 24 hours, thus extending its duration by six weeks and allowing parties time to redesignate.
The five Alliance Party MLAs said yesterday they would definitely not redesignate as Unionists, but the two Women's Coalition MLAs have not ruled it out. "In order to save the agreement we could be prepared to redesignate to Unionist or Nationalist, whatever was needed," MLA Ms Jane Morrice said.