Mr David Trimble and Mr Mark Durkan must win the support of a majority of both unionist and nationalist Assembly members to be elected as the North's First and Deputy First Ministers tomorrow.
While the support of a majority of nationalist members is guaranteed, majority unionist support is far from certain for the pair.
Of the 107 voting Assembly members 58 are designated unionist, 42 nationalist and seven other. The unionist camp is composed of 28 Ulster Unionists (UUP), 21 Democratic Unionists (DUP), three Northern Ireland Unionists (NIUP), three members of the United Unionist Assembly Party (UUAP), two Progressive Unionists (PUP) and one United Kingdom Unionist (UKUP).
If Mr Trimble and Mr Durkan win 30 of these unionist votes they will be elected. If 29 unionists vote for them and 29 against, they will lose, since a majority is required.
However, it is possible they could be elected with less than 30 votes if some unionists abstain, as only a majority of votes cast is necessary.
Assuming the 28 members of the anti-agreement unionist parties - the DUP, NIUP, UUAP, and UKUP - vote against Mr Trimble, the election is likely to hinge on how the PUP and the UUP's two dissident members, Mr Peter Weir and Mrs Pauline Armitage, vote.
A possible scenario is the election of Mr Trimble and Mr Durkan by 30 votes to 28 if the PUP (2) and all of the UUP (28), including Mr Weir and Mrs Armitage, vote for them. The pair would be elected by 29 votes to 28 votes if the PUP (2) and all but one of the UUP (27) vote for them, and one dissident Ulster Unionist, Mr Weir or Ms Armitage, abstains.
Mr Trimble and Mr Durkan will be defeated by 28 votes to 30 if all the UUP (28) support him but the PUP (2) vote against. However, this is considered unlikely as the PUP is expected to announce it will support Mr Trimble later today.
The picture is complicated by the fact the Women's Coalition (WC) has put forward a motion which, if passed, would allow them to redesignate from "other" to "unionist" in time for the election.
The addition of two extra pro-agreement unionist votes could have a decisive impact on the outcome. But it is possible one WC member may redesignate as unionist and the other as nationalist, even though this would limit Mr Trimble's chances of re-election. Under the standing order, redesignation can only occur "30 calendar days after notification" has been given to the speaker.
Before tomorrow's election the Assembly is due to hear the WC motion to amend the standing order so that redesignation "takes effect immediately after notification". For such a motion to succeed it need only attract 40 per cent of the nationalist and 40 per cent of the unionist vote.
The motion will not be heard tomorrow, however, if Mr Cedric Wilson finds the 30 signatures he needs for his petition of concern, which would delay it for 24 hours.