The split within the UK Unionist Party, which held five of the Northern Ireland Assembly's 108 seats, means that the Unionist cause is now represented by six separate factions within the new institution. Both of the new fragments remain opposed to the Good Friday Agreement.
Opinions differ, however, on whether the development will in the end represent a net gain or a net loss for the pro- agreement forces that dominate the assembly. The agreement needs a stable and substantial section of Unionism to prop it up, but with such confusion and disarray in the ranks, uncertainty is the order.
In yesterday's split, four UK Unionist assembly members abandoned their leader, Robert McCartney, announcing they were forming themselves into the Northern Ireland Unionist Party. While Mr McCartney seems to command the support of his party's grass roots outside the assembly, within the chamber itself he will become leader of a party without any other members.
The party has in recent times been closely aligned with the Rev Ian Paisley's Democratic Unionist Party, campaigning vigorously for a "no" vote in last year's referendum on the Good Friday Agreement.
The split was accompanied by a fair amount of acrimony. Mr McCartney accused the four of an act of "political infamy", saying they had committed a fraud against the electorate and challenging them to resign their sets and fight by-elections. He said the dissidents had little or no support and denied their claim that he was a despot intent on dragooning them into a boycott of the assembly.
The dissidents said in a statement that they were opposed to Mr McCartney's "insistence that we blindly acquiesce in his exit strategy from the assembly".
They said that would weaken the anti-agreement forces, adding: "For elected members to withdraw from the assembly on the personal whim of a party leader at a time of maximum crisis for the Union would be an act of gross political irresponsibility."
Mr McCartney is one of Northern Ireland's most familiar political figures, taking a prominent part in Unionist politics since the early 1980s. He is Westminster MP for North Down, having relinquished a profitable practice as one of Belfast's leading QCs to take up a full-time political career.
He left the Ulster Unionist Party after disputes in the 1980s, and more recently led his party out of the talks that led to the Good Friday Agreement.