Ulster Unionist leader Robin Swann has accused the DUP of "wasting an opportunity" over the failure to have an agreed victims' candidate in the West Tyrone by-election.
The MLA said attempts to secure a candidate to contest the Sinn Fein-held seat didn't happen because of a "lack of engagement" from other parties.
He was speaking at the party's West Tyrone association, days before thousands go to the polls.
The by-election was caused by the resignation of Sinn Fein's Barry McElduff, who was heavily criticised after posting a video of himself with a loaf of Kingsmill bread on his head on the anniversary of the 1976 massacre of 10 Protestant workmen.
Mr McElduff denied it was an intended reference to the sectarian murders by republican paramilitaries near the south Armagh village of Kingsmill exactly 42 years earlier.
The outrage over the social media post sparked talk of an agreed victims' candidate running.
Mr Swann, however, claimed it was rejected by the DUP.
Sinn Fein has held what is a safe seat for 17 years. It is being challenged by the UUP, DUP, SDLP and Alliance.
The republican party is expected to retain the seat, as Mr McElduff received 22,060 votes in last year's election, well ahead of the DUP's Thomas Buchanan on 11,718 votes.
In a speech on Saturday in Edenderry, Mr Swann said: "This was an election that should have had a non-partisan, cross-community candidate.
"This is an election where there could and should have been someone standing that all the parties could have supported as a victims' candidate.
"I can tell you that it was through no lack of effort from the UUP that it didn't happen. I was the first politician to call for it. We tried to get support for such a candidate, but it was due to a lack of engagement from other parties that it didn't happen.
"Only they can give the real reasons for their lukewarm response to my suggestion. And don't let anyone in the DUP tell you that they weren't approached or asked about it.
"I stand over what I said before. I directly approached Timothy Johnston, the chief executive of the DUP.
"Only the DUP can explain how their internal lines of communication work. It is absolutely, categorically untrue for any member of the DUP to claim that their party wasn't approached. It was a wasted opportunity."
A DUP spokeswoman said: "This is a distraction from the decision people have to make on Thursday.
"The DUP wanted to engage sensibly but the UUP had no interest despite originally calling for an agreed candidate.
"Immediately after Barry McElduff resigned, Tom Buchanan offered to step aside if such a candidate came forward. Nobody did.
"It was clear the UUP had already decided upon a party representative.
"Robin Swann is trying to deflect from this fact.
"It is now for the people of West Tyrone to decide who will represent them at Westminster."