Mr Jeffrey Donaldson's resignation from the Ulster Unionists has been greeted by the Rev Ian Paisley, who claimed it reduces the UUP to "a rump" within unionism.
Officially at least, the UUP is reacting to the Lagan Valley MP's decision "with regret", although not with surprise. Some within the party are pleased he has gone.
The resignations of Mr Donaldson and two anti-agreement colleagues, Ms Arlene Foster and Ms Norah Beare, would reduce the UUP total in the Assembly to 24 if suspension ended.
However, Mr David Trimble has said: "Questions like this are all academic," and pointed to the fact that there is no prospect of Stormont members being recalled. He believes the DUP has no proposals that would lead to a restoration of powers to Stormont and that direct rule from London will continue for the foreseeable future.
A UUP statement claimed that Mr Donaldson, who has been Mr Trimble's most persistent critic since the signing of the Belfast Agreement in 199, had "lost his way" politically.
Mr Donaldson said the UUP had changed and he had been consistent.
Mr Donaldson, a member of the Ulster Unionists for more than 20 years, was a close ally of former South Down MP, the late Enoch Powell, and also of Mr Trimble's predecessor as UUP leader, Lord Molyneaux.
The DUP is understood to be keen to have him join its ranks and has offered him a place on the party negotiation team.
Mr Donaldson said on Thursday that he would be joining the DUP's negotiating team in the New Year review of the Belfast Agreement.
He told the BBC yesterday: "I've got to find another way of getting to the negotiating table, so we can work for the changes that will deliver a better agreement. There is no point in continuing with what is an increasingly acrimonious debate within the Ulster Unionist Party which, quite frankly, is futile because they are not going to change their attitude.
"Therefore, I will work with other unionists who see and listen to what the electorate has said and are going to work for that better agreement."
The SDLP said the move underlined the anti-agreement stance of most unionist Assembly members.
The Alliance leader, Mr David Ford, used the resignation to support his party's demand for reform of the Assembly's voting procedures to enable the selection of a First and Deputy First Minister and an Executive.
The resignation spares the Ulster Unionists from returning to the ultimatum party officers delivered to Mr Donaldson last week. They ordered him, following a majority vote supported by Mr Trimble, to follow the party line on the Belfast Agreement or quit the party.
The officers were due to consider Mr Donaldson's situation again on January 9th.
In theory, Mr Donaldson and his supporters could have gathered the required signatures to force yet another meeting of the party's ruling Ulster Unionist Council and a fresh challenge to party policy. One senior party member told The Irish Times that the required signatures were "kept on headed notepaper - all Jeffrey has to do is put in the date".
In any case, the UUC is due to reconvene in March for its annual general meeting.
Dr Paisley said that Mr Donaldson's departure from the UUP was a momentous decision that would deal a hammer blow to that party, leaving them little more than a rump of unionists.