Ulster Unionist party leader Mike Nesbitt has said the decision to form election pacts with the DUP was the "best possible" arrangement.
He was addressing members at the party's spring conference at the Dunadry Hotel.
Mr Nesbitt said the decision to work with the DUP was "because it is in the best interests of Northern Ireland to maximise the number of pro-Union MPs returned to Westminster".
The DUP and UUP struck a historic deal which will mean a single unity candidate standing in four constituencies across Northern Ireland.
Following six months of secretive talks, the agreed candidates are Gavin Robinson in East Belfast, Tom Elliott in Fermanagh and South Tyrone, sitting MP Nigel Dodds in North Belfast, and Danny Kennedy in Newry and Armagh.
"Without co-operation, there was a real danger that our capital city would have no pro-Union MP after the May 7," he said.
"I know this understanding does not maximise the pro-Union potential, in Belfast or Northern Ireland-wide, not least because the DUP would not discuss South Belfast in a practical manner.
"They had other priorities. So, the understanding is not what Peter Robinson wanted any more than it is what I wanted, but politics is about the art of the possible, and this arrangement was the best possible, given the different direction the DUP was approaching from."
"We enter the Election with no MPs. Will we emerge with a ticket back to the green benches?
"We have some exceptional candidates, some of whom have been selected and endorsed for many months now.
"Over the next 40 days, let’s get behind them and the Party. Let’s remember a lot of decisions in politics are judgement calls, and none of us will agree with everything the Leadership decides. The important thing is how we handle ourselves."
Meanwhile DUP leader Peter Robinson said split votes "cost unionism seats and lost seats means lost influence at Westminster".
The First Minister was speaking at his party's spring conference in Templepatrick.
He said: "Whose interests are ultimately served if two or more unionists divide - mostly over matters of detail rather than matters of principle - and allow a non-unionist to be elected?
"It’s simply not in the interests of unionism.
"In no negotiation will everyone achieve everything they want but I believe that the deal I did with Mike Nesbitt is good for unionism and good for Northern Ireland.
"In Fermanagh and South Tyrone and Newry and Armagh I want to pay tribute to our associations who have been asked to stand aside. But make no mistake in twelve months time, at the next Assembly election, the wider interests of unionism will be best served in these two constituencies by voting for the DUP candidates.
"Though, again I say it, I hope in that election too we can have a voting agreement that ensures voting preferences go to other unionist candidates.
"Whatever anyone may think of the balance of the pact, that debate and discussion has ended."