Northern Ireland’s devolved government will be endangered if the election does not produce change, Colum Eastwood said.
The SDLP leader appealed for the 50% who normally did not vote to help snub the strained DUP and Sinn Fein partnership and deliver power-sharing in spirit through the SDLP and UUP.
UUP chief Mike Nesbitt has faced criticism from within his own party after he said he would select the nationalist party as his second preference on the Assembly election ballot paper on Thursday.
The SDLP chief said: “I genuinely do think devolution is under enormous stress and threat and I just think it is worth fighting for.
“We are in a very dangerous place and I think the result on Thursday will determine what kind of future we have.”
The SDLP leader warned: “I think if we return the same people in the same way with the same number of seats and the same power bases, then it is going to be very difficult to form a government.”
Mr Eastwood said the idea that the UUP and SDLP were supposed to have a common platform after eight months was nonsense.
He withheld backing from Ulster Unionist proposals like a common enterprise zone covering the whole of Northern Ireland.
He said: “We are two political parties, all we are saying is we don’t have an electoral pact, but what we are saying is we have an attitude that means we can work together.”
The key to change would be encouraging the 50% who do not vote to come out.
“Einstein talked about repeating the same thing over and over again, it is insanity.
“If they want something different, they should come out and vote for something different. How much worse can it be?
“I think there is an opportunity if people vote for it, but it is really in the lap of the gods and in the lap of people.”
Brexit negotiations made it even more important Northern Ireland was not left voiceless, the Foyle candidate told a business event in South Belfast organised by the Northern Ireland Chamber of Commerce and Industry.
He claimed the border should be around Great Britain rather than on the island of Ireland.
Mr Eastwood apologised to business leaders for the delay in delivering a reduced rate of corporation tax while Tory cuts in Britain made the measure less attractive in Northern Ireland.
He said political uncertainty around Europe had created the “perfect storm”.
“We must take back control of the powers we continue to hold in an uncertain economic climate. We must begin to create our own certainty in Northern Ireland.”