Unionists have been engaging in "quiet conversations" with an SDLP forum on Irish unity for the past 18 months, the party's leader Colum Eastwood has said.
He said people from "very different backgrounds" have been meeting with the New Ireland Commission, which was set up to discuss future constitutional arrangements on the island.
Mr Eastwood described how the civic forum, which is moving into a "public phase" of work, is central to the SDLP's "new mission" around "building a movement for a new Ireland".
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He argued his party is best placed to "convince the unconvinced" as Sinn Féin "can't reach into communities that we can reach into" while Alliance "don't seem to have a view".
In an interview with Belfast Live ahead of the SDLP's annual conference this weekend, Mr Eastwood disputed claims that unionists will not engage in the debate.
He said: "When I hear all the time people saying that unionists aren't going to engage in the conversation about the future of the island, well I can tell you for a fact that's not true, because they've been engaging with us over a period of 18 months of quiet conversations.
"We're getting people coming to meetings from very different backgrounds, and telling us what they think. That's the work that we're engaged in, because we really believe if we're serious about building the case for a united Ireland, we have to bring people with us.
"We have to understand what their fears are, what their concerns are."
He added: "I mean, we could have, you know, huge public rallies and trumpet them all around the place, but we actually want to win in the end and we want to bring people with us.
"That's the mission of the SDLP. That's what we're engaged in. And I think you'll see more and more activity now that we've moved into the public phase."
Mr Eastwood said his party is "best placed" to make the arguments for a united Ireland.
He added: "Sinn Féin will do their work, and that's fine, but they can't reach into communities that we can reach into. They can't convince the unconvinced.
"And Alliance of course won't take part because they don't seem to have a view on the future of the island, and that's for them to explain.
"But we have a very clear role now - moving beyond Brexit, moving into the conversation about the future of the constitution.
"I think we have a clear role in being the convincers that the kind of Ireland that we want to see is based on reconciliation, and we want to bring people with us."
Mr Eastwood dismissed recent opinion polls which suggest SDLP support has dipped further since elections last May when the party lost four MLA seats, dropping from third-largest party at Stormont to fifth-largest.
But the Foyle MP acknowledged the current "political context is very difficult for us".
Mr Eastwood said DUP leader Sir Jeffrey Donaldson "told people that we couldn't have a nationalist First Minister" by refusing to say during the Assembly election campaign if they would share power alongside a Sinn Fein First Minister.
He added: "And people responded, and that is still an issue that hasn't been resolved. That mandate hasn't been allowed to be fulfilled. So I think that is where people are at right now.
"But that's a very temporary position. I'm very confident actually that this weekend at our conference we're going to set out a new mission for the SDLP, which is about building a movement for a new Ireland. Doing the hard yards to convince people that the future is best placed within a new Ireland."
Mr Eastwood said he believes the party will "do well" in May's council elections, dismissing claims the party could suffer significant losses.
He added: "I think people will be very happy with the slate of candidates that we have, and people will be surprised at the result that we get."
The Foyle MP insisted this would "absolutely not" be his last SDLP conference as party leader despite recent electoral pressures.
The party recently announced that Jackie Coade, a former Alliance Assembly candidate who quit over how it handled her complaint of verbal abuse, is standing for the SDLP in the upcoming election.
Mr Eastwood was unable to confirm whether Paul McCusker, a high-profile SDLP councillor in North Belfast, will be a candidate for the party in May.
The SDLP leader said a tweet from the party which announced that Mr McCusker and Charlotte Carson would run in Oldpark was later deleted "because it hadn't been ratified yet".
Mr Eastwood praised Mr McCusker as "one of the most talented community activists that I've ever seen".
"I really hope Paul is going to be one of those candidates because he's a tremendous asset, not just to the SDLP but to Belfast City Council," he added.
Mr Eastwood defended how the SDLP had voted against a council rates increase in Belfast but joined other parties in supporting a rates rise in Derry and Strabane.
He said: "In every single council where we have representatives, we wanted no increase at all. But sometimes that's just not possible."
Mr Eastwood also said he is "not interested in mergers" after Irish Labour Party leader Ivana Bacik refused to rule one out.
In a recent interview she told the Sunday Independent there have been "extensive discussions" with the SDLP on collaboration on an all-island basis.
Mr Eastwood responded: "We work with all political parties. We work very well with Irish Labour, of course, because we're both members of the Party of European Socialists, but there's no discussion about a merger or anything else."
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