David Cameron and Sir Reg Empey forged an alliance in 2008
Two former Conservative candidates who withdrew their nominations have said it was partly because of a possible deal between the Ulster Unionists and DUP.
The Tories have made an alliance with Ulster Unionists which would see the two parties field a joint candidate in the general election in NI.
Sheila Davidson and Peter McCann said they withdrew because they feared the Ulster Unionists would do a separate deal with the DUP.
They said they still backed the Tories.
There has been speculation that the pair, who are both Catholic, had left in protest at talks hosted by the shadow secretary of state Owen Paterson earlier this month.
The Conservative MP invited the DUP and the Ulster Unionists to the discussions at Hatfield House in England. He said the purpose was to discuss how to strengthen the Northern Ireland Executive.
However the talks were criticised by nationalists, including the SDLP deputy leader Alasdair McDonnell, who accused the Tories of attempting to create a pan-unionist front and "playing the Orange card".
Sheila Davidson said that while Owen Paterson had been a "diligent" shadow secretary of state, she had some concerns about how the talks would be viewed.
She said: "He thought he was doing the right thing in bringing together two parties to discuss how they might join together to help policing and justice move forward as a devolved issue in NI.
"I would maybe have had a a little bit more sensitivity as to how that would be perceived by the wider community coming into the public domain."
Mr McCann said that he took his decision in the light of what he believed the Ulster Unionists were doing.
"Our sister party seemed to be intriguing into a public link-up with the Democratic Unionists. That was the point where I decided that the process was not going where I wanted.
"I am a member of the Conservative Party and I would never enter into any pact or deal where the DUP were part of that."
Both candidates emphasised that a possible deal between the Ulster Unionists and the DUP had not been the sole reason why they withdrew their nominations.
Both parties were supposed to make their own nominations for each of 18 constituencies. A joint-committee would then select one candidate to go forward to fight the election.
Ms Davidson said this process "stalled and stalled and stalled" to the point of frustration.
She said: "We asked on Friday: can you draw a line in the sand and say that we have to by such and such a date or any date? But the party has taken a decision that it is going to wait and see when the UUP partners are ready".
Mr McCann said that thoughts of a DUP-UUP deal had delayed nominations even further.
"What became apparent after last week when this unionist unity approach was coming was that the stalling was going to go on even further because this became more important."
Both former nominees said they still believed that the Conservative vision of non-sectarian politics was the right way forward for Northern Ireland.
On Monday, the head of a grassroots Conservative group said he believed the alliance between the Conservatives and the Ulster Unionist had gone on too long and should be ended.
Chair of the Campaign for Conservative Democracy, John Strafford, said there had been undue delay in selecting joint candidates and criticised the talks with the DUP.
Writing on the political blog Slugger O'Toole Sir Reg Empey said that he was sorry Mr McCann and Ms Davidson had stepped down from the process.
However, he said it was an internal matter for the Conservatives.
He added: "That three Conservative Party candidates have decided to withdraw from the process is regrettable.
"All three are extremely capable individuals who would have added much to the partnership between our parties and to the political life of Northern Ireland."