The DUP has criticised the Traditional Unionist Voice (TUV) for urging supporters to vote for the Conservatives and Unionists in certain constituencies.
TUV leader Jim Allister said he would ask supporters to switch their support in areas where he had not fielded a candidate for the election.
But the Democratic Unionists claimed the move could help Sinn Féin in key constituencies.
The DUP's Sammy Wilson said the move would boost the prospects of republican candidates John O'Dowd, in Upper Bann, and Gerry Kelly, in North Belfast.
Mr Wilson said: "This is the clearest demonstration that TUV is about opposing the DUP rather than promoting unionism.
"This advice will have John O'Dowd and Gerry Kelly jumping for joy.
"In North Belfast and Upper Bann only Nigel Dodds and David Simpson can ensure these seats are maintained for unionism, yet Jim Allister says people should vote Tory.
"Grassroots unionists will see through this TUV tactic... They will go to any length, regardless of consequences, to attack the DUP."
Mr Allister had told the BBC that he credited the UUP with having voted against the devolution of policing and justice powers to the Assembly.
PA