Colum Eastwood was speaking at the launch of his party's manifesto, 'Taking our seats, taking a stand,' at an arts centre in south Belfast.
The party is defending three seats - Foyle, South Belfast and South Down in the general election.
Mr Eastwood used the launch to discuss the prospect of a border poll.
"While others were waving banners, the SDLP made the prospect of a successful unity referendum much more possible," he said.
He stressed that "a border poll is no longer solely the project of Irish nationalism but of pro-European internationalism".
The SDLP leader added: "A unity referendum now has a much broader reach, offering us a return to the European Union (EU) as a sovereign country."
Answering a question from journalists about the prospect of a united Ireland referendum, he said it would only happen "after the UK has left the EU".
He said he wanted a "border poll we can win".
Colum Eastwood said next week's election was all about Brexit and the poll in June had been called by Theresa May "to enforce a future to which we haven't given our consent".
He said the DUP and Sinn FÃ©in "are going into this election offering our people permanent Tory governance".
The SDLP leader added that "the only way to take a stand in this election is to take our seats".
In a direct attack on Sinn FÃ©ins policy on abstentionism he said "shouting from the sidelines will make no difference".
Colum Eastwood told supporters that his MPS would "not sit at home and abdicate responsibility "
The Foyle MLA also told supporters that his party's manifesto "stands up against borders, division and cruel crippling cuts".
Understandably the SDLP manifesto centres much on the issue of Brexit but the document also examines economic and educational areas and focusses on human rights and the issues of racism and immigration.
The party said theirs were the only MPs from Northern Ireland to vote against the Welfare Cap in 2014.
They said they will continue to support women who are campaigning for fair pensions.
The manifesto includes a commitment for a pay rise for nurses, midwives and other medical staff.
The manifesto states there should be greater north-south health care and the party wants a "joined up approach to ambulance service provision in the border areas".
The party also states that in order to tackle a housing shortage there should be 3,000 new social homes in Northern Ireland.
Other manifesto pledges include a call for an Irish Language Act, city deals for local councils, opposition to fracking and the party say they will challenge any attempts to devalue human rights at home and abroad.
The leader of the SDLP has said a referendum on an united Ireland is "no longer solely the project of Irish nationalism".