An SDLP MLA has said the party is not going away after details emerged of a new partnership with Fianna Fáil.
The SDLP and Fianna Fáil are expected to announce a schedule for a closer working relationship and policy discussions on Thursday.
West Tyrone MLA Daniel McCrossan said "this is a big change".
He said he had concerns, but they had been outweighed by the fact "this was the right way to go".
"This is something that some will be uncomfortable with and some will not be quite sure about, but this is the right move," he added.
"This is about moving to an all-Ireland dimension as one movement.
"A new political movement across the the island between two parties and an understanding that we must meet the big challenges that we are facing together and I'm sure that the negotiations over the past few months have clearly laid down a platform to achieve that."
Fianna Fáil sources have downplayed suggestions of a merger or an alliance.
The party's Brexit spokesperson Lisa Chambers told BBC Radio Foyle: "We are not merging, it is a partnership.
"We will retain our distinct identities."
The parties are due to unveil a shared policy platform dealing with high level themes.
SDLP sources have also stressed what has been under discussion is a partnership rather than a merger.
At a meeting of the party's management committee on Tuesday night, leader Colum Eastwood briefed members and MLAs about the talks.
He told them there would not be a merger and that prospective candidates for the local government elections in Northern Ireland in May would still stand under the SDLP banner.
SDLP sources said party branches would be given more information on Wednesday and a special party conference would be held in mid-February to vote on the "formalised partnership".
In October last year, Fianna Fáil headquarters were at loggerheads with some senior members of the party after they appeared to announce a Fianna Fáil candidate in Northern Ireland.
At a meeting in Omagh, County Tyrone, it was announced that councillor Sorcha McAnespy would be standing as a candidate in Northern Ireland's local council elections.
But shortly afterwards Fianna Fáil headquarters issued a statement reiterating that the party had made "no decision" on contesting the election.
Fianna Fáil Senator Mark Daly was at the event in Omagh and was named as Ms McAnespy's campaign manager.
He was subsequently sacked as the party's deputy Seanad (Irish senate) leader and spokesperson for foreign affairs.
Some SDLP members have previously said they would oppose any merger, including South Belfast MLA Claire Hanna, who told BBC News NI she would not join Fianna Fáil.