The Ulster Unionist Party leader, Mr David Trimble, has congratulated Mr David Burnside, a London-based PR executive and perceived hardliner, on winning the party's candidature for the South Antrim by-election.
On Monday night, Mr Burnside defeated Mr David Campbell, who is Mr Trimble's chief of staff and a staunch pro-Belfast Agreement politician, by 90 votes to 70.
The by-election, made necessary by the death of Mr Clifford Forsythe, will be held in the autumn and is likely to turn into a battle between Mr Burnside and the DUP's Rev William McCrea.
Playing down differences with Mr Burnside, Mr Trimble said: "David and I have worked together for many years on modernising the presentation of the party and I look forward now to the whole association and the party rallying round to reject the DUP - a party that has done more harm to the image of unionism and the future of Ulster than any other factor in the last 30 years."
Following his victory, Mr Burnside said he had supported Mr Trimble in the leadership contest with the Rev Martin Smyth and had only ever disagreed with him over "tactics, not end objectives". He was only interested in winning South Antrim for the UUP, not in winning the party's leadership, Mr Burnside added.
"I will make a realistic attempt to bring the Yes and the No camps together . . . If South Antrim proves that we can unite them I believe we will see a great recovery of electoral support for the Ulster Unionist Party," he said. The UUP MLA for Mid-Ulster, Mr Billy Armstrong, joined Mr Trimble in congratulating Mr Burnside, adding: "David Burnside has many constructive ideas and he exhibits excellent capabilities and this will endeavour him to win the seat as Ulster Unionist Party MP for South Antrim."
The Northern Ireland Unionist Party MLA for the area, Mr Norman Boyd, said he was disappointed that the UUP had selected a pro-Agreement candidate, who was, moreover, based in London, with little knowledge of the constituency.
Despite having made some critical noises in the past, Mr Burnside wanted to see the full implementation of the Belfast Agreement, which had been a disaster for unionism, Mr Boyd added.
Mr Burnside's main challenger for the seat, the DUP's Rev McCrea, described him as a "John Taylor, Mark Two".
Instead of uniting the Yes and No factions within the UUP, Mr Burnside was straddling both sides of the agreement argument and could be trusted by neither, Mr McCrea added.
"If the people of South Antrim want another John Taylor then Mr Burnside is the man to vote for. A man whose position is subject to change with the political landscape."