David Trimble: Needs votes of all UUP colleagues
The Ulster Unionist Party leader resigned in July because of the absence of IRA decommissioning and then withdrew his ministers from the executive two weeks ago to put further pressure on republicans.
However, following the IRA move to destroy some of its weapons last week, he decided to bring his party back into government, a decision that has been backed by his party's executive.
But Mr Trimble's prospects of returning as first minister received a blow on Thursday when a hardline UUP assembly member said she would not support his nomination.
He requires the votes of all his UUP colleagues, as well as other pro-Good Friday Agreement unionists, to win the vote.
Pauline Armitage said she had grave doubts about the IRA's gesture on decommissioning and the general direction of the peace process.
Meanwhile, the Women's Coalition has succeeded in changing the rules of the assembly to re-designate the affiliation of its two members, in an attempt to give Mr Trimble at least one extra unionist vote.
Jane Morrice has re-designated herself as a unionist, while Monica McWilliams has changed to a nationalist.
The loyalist Progressive Unionist Party has said it will support Mr Trimble. However, the party's two votes are still not enough to secure the future of the assembly.
David Ervine of the PUP said it was crucial that unionists remained united.
Re-designate
He told BBC Radio Ulster on Friday that the situation within the Ulster Unionist Party was potentially very damaging.
"This is so important for unionists," he said. "If unionism does what I think it may do today, it would be committing hara-kiri."
The vote of another dissident Ulster Unionist, Peter Weir, could decide the outcome of the devolved institutions.
Mr Weir has not yet declared whether or not he will support Mr Trimble.
Senior Ulster Unionist Lord Kilclooney (John Taylor) said the party should "run with David Trimble" to ensure decommissioning continued.
"If we don't run with him today, decommissioning will stop, there will be a collapse in the political institutions in Northern Ireland, the economy will be damaged and I fear violence will begin once again," he said.
He said he agreed with Mr Ervine's analysis that the party would be committing political suicide unless it backed the party leader.
Sinn Fein have called for fresh assembly elections if the devolved government fails to fill the top two assembly posts.
The first and deputy first ministers will have to gain a majority of support within both the unionist and nationalist blocs in the assembly to be elected.
The leader elect of the nationalist SDLP, Mark Durkan, who is hoping to win the assembly's backing to take the deputy first minister's post, said all Ulster Unionists must support Mr Trimble at the election.
Alliance leader David Ford has said his party would not consider re-designating as unionists in order to secure Mr Trimble's re-election.