Ulster Unionist leader Mike Nesbitt has warned that Belfast could have been left without a pro-Union MP if his party hadn't done an election deal with the DUP.
He told the UUP spring conference that the pact in four Westminster seats was the "best possible" arrangement.
"Without co-operation, there was a real danger that our capital city would have no pro-Union MP after May 7," he said.
But he added that the pact's potential had not been maximised, as the DUP "would not discuss South Belfast in a practical manner".
"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," he said.
The historic deal will see a single unity candidate standing in four constituencies across Northern Ireland for May's general election.
Following six months of secret talks, the agreed candidates are the DUP's Gavin Robinson in East Belfast, the UUP's Tom Elliott in Fermanagh and South Tyrone, the DUP's Nigel Dodds in North Belfast and the UUP's Danny Kennedy in Newry and Armagh.
Announcing the pact, Mr Nesbitt and DUP leader Peter Robinson said agreement in South Belfast and Upper Bann had not been possible.
Mr Nesbitt said there was a "potentially once-in-a-lifetime opportunity" for a unionist to win Newry and Armagh, although he conceded it would be difficult.
"Our support for the DUP in East Belfast should ensure an additional pro-Union MP for the city of Belfast in the next mandate," he said.
After the announcement, outgoing Sinn Fein MP Conor Murphy called on the SDLP to consider a nationalist pact, saying it should put progressive politics ahead of its own "narrow party interests".