Sinn Fein and the SDLP should form a united nationalist party, Declan O'Loan has said.
The two parties' supporters interchange their votes, the North Antrim SDLP MLA added. He said the SDLP could command second preference ballots across the community.
The DUP's Jimmy Spratt accused him of hypocrisy after the SDLP said fielding a Fermanagh and South Tyrone unionist unity candidate was a sectarian carve-up.
Mr O'Loan said: "I believe that a major realignment of northern nationalism is now called for and I think that this means the formation of a new single nationalist party.
"The values of the SDLP and its ability to command respect and at least second preference votes across the whole community are not something that should be lost.
"In the interests of achieving Irish unity, that respect is vital."
The SDLP retained three seats at Westminster to Sinn Fein's five. It rejected Sinn Fein's calls for Fearghal McKinney to stand aside in Fermanagh and South Tyrone, saying it would not participate in sectarian politics. In South Belfast, SDLP MP Alasdair McDonnell won convincingly after Sinn Fein's Alex Maskey stood aside.
Mr O'Loan said: "The election was marked by a high level of tactical voting within the nationalist electorate, and indeed between nationalism and unionism. Many nationalist voters are willing to exercise their votes interchangeably between the SDLP and Sinn Fein. Equally it is clear that there remains solid support for what the SDLP stands for and that support is not going to change."
Mr Spratt, who lost out in South Belfast, said: "Alasdair McDonnell said it was totally sectarian when it was suggested (fielding a single unionist candidate) in South Belfast. Calling the pot black now springs to mind. If it was sectarian from a unionist perspective then it must be sectarian from a nationalist perspective. It is wonderful how fortunes change."
Sinn Fein MLA John O'Dowd also said the SDLP was changing its views. "At times the mixed messages coming from the SDLP on the issue of maximising nationalist representation are completely confusing," he added.