Belfast City Council could elect its first Sinn Fein lord mayor tonight if the Alliance party agrees. Alliance is due to announce its decision at a press conference this morning.
Its council leader, Dr David Alderdice, last night refused to say whether the party would be voting for the Sinn Fein candidate, Mr Alex Maskey.
"It has been a difficult decision," he said. "But after lengthy consultation with our own members, and with Sinn Fein, the SDLP and other parties, I am confident that we have decided on the right course of action."
Sinn Fein is the largest party on the council, and the SDLP has said it is supporting Mr Maskey. However, he needs the votes of the three Alliance councillors who hold the balance of power in City Hall to secure the mayorship. It is understood that no unionist councillors will vote for him.
Mr Maskey, who has survived several loyalist assassination attempts, was elected in 1983 and is Sinn Fein's longest-serving member of the council. There has been an SDLP lord mayor, Mr Alban Maginness, but Sinn Fein has never held the position.
It is understood that Alliance is concerned about the reaction of its voters to supporting Sinn Fein in the mayoral contest. The party's vote has already fallen in the Westminster and local government elections.
However, it would be difficult for Alliance not to back Mr Maskey, given that it supports Sinn Fein Ministers in the North's Executive. Sinn Fein has argued that Alliance has a democratic duty to support its candidate, while unionists insist that Sinn Fein should not hold political power until there is Provisional IRA decommissioning.