A decision by Ian Paisley's DUP to contest the Westminster seat for the Fermanagh/South Tyrone constituency could mark the end of unionist electoral co-operation for marginal seats.
The constituency is predominately nationalist, but the nationalist vote has always been split between the SDLP and Sinn Féin, allowing the UUP's Ken Maginnis to hold the seat. In 2001, Sinn Féin's Michelle Gildernew polled strongly enough to take the seat by less than a hundred votes.
Ulster Unionists have now claimed the DUP's decision to field Arlene Foster guarantees Michelle Gildernew will hold onto her seat.
The decision could have a knock-on efect in other constituencies such as North Belfast.
But DUP chairman Maurice Morrow hit back, accusing the Ulster Unionists of launching a ``nasty, personal and abusive attack'' on Mrs Foster who defected from their party in January.
Democratic Unionists chose Mrs Foster, who is an assembly member for Fermanagh and South Tyrone, at a selection meeting on Friday night.
A UUP spokesman accused her today of using his party to achieve her current position as an assembly member, only to desert them.
Ulster Unionist Assembly member Tom Elliott also said: ``We can all remember how the DUP supported (independent unionist) Jim Dixon in the 2001 Westminster election, which helped secure the victory for Sinn Féin.
``After almost three years of having a Sinn Féin MP for Fermanagh and South Tyrone has helped or been in any way beneficial to this constituency, this province or indeed the unionist cause.
``I am disappointed that Mrs Foster is being used by her new friends following her walkout from the UUP to benefit republicans.''
Former DUP social development minister Maurice Morrow said: ``It is the (David) Trimble led UUP who have surrendered to Sinn Féin/IRA over the course of the last six years.
``Unionists in this constituency and across Northern Ireland have rejected the policies of pro-agreement unionists at the assembly election.''
``The Trimble analysis has been rejected. Anti-agreement unionist candidates outpolled Trimbleite candidates by 13,649 votes to 8,291 votes.''
Mrs Foster narrowly missed out on becoming the UUP's candidate in Fermanagh and South Tyrone in the last Westminster election, losing out to James Cooper.