A leading Ulster Unionist has called on the DUP to stand aside in the upcoming Mid Ulster by-election, which will be held on March 7.
Former MLA and Westminster candidate Billy Armstrong said that the UUP had stood aside for the DUP in the past.
"It's our turn now," he said.
Both he and DUP constituency chairman Paul McClean confirmed that efforts to find a unionist candidate outside the party structures had so far failed.
"Nobody has been found or approached as far as I am aware," Mr Armstrong, UUP constituency treasurer, said.
And Mr McClean confirmed: "There are no names in the frame at this point."
Both party leaderships are committed to continuing the search – but it must end by February 25 when nominations close.
With the deadline looming, both parties will be forced to start their own selection procedures and printing election material straight away, so as not to be caught out on election day.
"We can put it on hold if an agreed candidate is found but we have to get the ball rolling," a UUP source said.
Mr Armstrong's daughter Sandra Overend is a sitting UUP MLA and is widely expected to be the UUP candidate.
Ian McCrea, the constituency's only DUP MLA, is expected to be selected by his party.
Mr McCrea's father Rev William McCrea was defeated as MP for Mid Ulster by Sinn Fein's Martin McGuinness in 1997 after holding it since 1983. Before that it was held by Jack Dunlop, a unionist unity candidate since 1974.
The UUP supported Rev McCrea in a 1986 by-election and in every subsequent contest until he lost to Mr McGuinness.
"We will work at it and exhaust the process till we see if we can get an agreed candidate, but it may be impossible to achieve," Mr Armstrong said. "If that doesn't work it is then our turn to put forward a candidate.
"In my younger years we all supported John Jack Dunlop and in later years we all supported William McCrea, and we didn't look any further for a candidate of our own.
"Maybe, in fairness, the tables should be turned the other way so that an Ulster Unionist could stand this time and the DUP and other unionists could back that candidate," he added.
Last month Simon Hamilton, the DUP's director of elections, did not rule out supporting an Ulster Unionist, although he said that a non-party candidate would be preferable.
However, Ian McCrea seems to reject a UUP candidate, telling a local paper that he disagreed with a statement by UUP leader Mike Nesbitt that "any unity candidate must display 'Ulster Unionist' values".
Mr Nesbitt has ruled out supporting a DUP candidate.
Whatever unionists do, most observers believe Sinn Fein, who took 49.5% of the vote in the 2011 Assembly election, will retain the seat.
They will field Francie Molloy, who is already canvassing.
Story so far
Martin McGuinness left his seat as Mid Ulster MP on January 2 to concentrate on his role as Deputy First Minister. The chief whip in Westminster moved a writ for a by-election to be held on March 7. The seat has a nationalist majority and both unionist parties are trying to find an agreed candidate.