Ulster Unionist Party leader Mike Nesbitt has called on two Assembly members who resigned from the party to step down from their Stormont positions.
The resignations of John McCallister and Basil McCrea last week has reduced the number of UUP members at Stormont to 13.
This leaves the party with fewer members than the SDLP for the first time in its history.
However, Mr Nesbitt has denied that the party is in crisis.
The former television presenter took over the leadership of the UUP last March.
He is struggling to stop the slide in the fortunes of what was once Northern Ireland's largest political party.
Mr McCallister and Mr McCrea resigned over the decision to field a joint candidate with the DUP in a Westminister by-election next month.
The pair see themselves as middle-ground, pro-Belfast Agreement unionists and have had policy rows with Mr Nesbitt for several months.
As a result of the resignations, the UUP is now the fourth largest party at Stormont.
The Democratic Unionist Party has 38 seats, Sinn Féin has 29 and the SDLP has 14 seats.
Mr Nesbitt said he feels liberated by recent events and called on Mr McCallister and Mr McCrea to resign their Assembly seats and seek a mandate through a by-election.