The crisis within the Ulster Unionist Party (UUP) has deepened after the resignation of two more members.
Basil McCrea and John McCallister stepped aside when the party decided to run a unionist unity candidate with the DUP to fight the Mid-Ulster Westminster by-election on March 7.
Leader Mike Nesbitt said the departures were a matter of huge regret.
They may join fellow former Ulster Unionist David McClarty in a new centre-ground grouping at Stormont.
Mr McCrea said: "I will say to the people of Northern Ireland, if you want a type of politics that is progressive, pluralist and that is mutual respect then you need to stop voting for sectarian head counts and start voting for people that represent all of the people of Northern Ireland.
"This is not sour grapes; this is not self interest, this is a principled stand to do what is right and it will be in the hands of the electorate and the electorate alone to decide if we have made the right call."
Nigel Lutton, the son of a former RUC reservist shot dead by the IRA in 1979, will be the unionist unity candidate in Mid-Ulster. However, the strategy of closer co-operation with the DUP has divided the Ulster Unionist party.
Mr McCrea, Lagan Valley MLA, has been at odds with the UUP leadership and Mike Nesbitt for some time, having led a failed bid for the top position. He criticised the party over its handling of issues linked to the Union flag dispute and faced a disciplinary panel earlier this year, which cautioned him about recent media comments. He announced his resignation on the BBC's Nolan Show.
Mr Lutton will run for the Westminster seat vacated by Sinn Fein's Martin McGuinness last month. The Democratic Unionists and UUP said Mr Lutton received unanimous support. His main opposition will be Sinn Fein veteran Francie Molloy. After resigning, Mr McCallister, South Down MLA, said he believed in opposition at Stormont and confirmed the possible link up with other disaffected unionists.
"We have no idea if it is going to work. One thing Basil and I cannot be accused of is doing this for career advancement. It does put us in a difficult position but I would rather not be in politics and still have my integrity and the principles I believe in," he said.