The SDLP held its three Westminster seats but its rival Sinn Fein added two more seats to its cache.
It has now overtaken the SDLP to become the main nationalist party.
Tom Kelly, chairman of the SDLP's communications group, said on Monday his party had "called it wrong".
" The 'mo' stands for both momentum and mood, two aspects of the political climate that are not with the SDLP at this time "
Writing in the Irish News Mr Kelly also called for an "invigorated leadership" for the SDLP.
He said changes recommended in a strategy document, which emerged after an internal review of the party 18 months ago, needed to be completed.
"We need to be brave enough to complete the change process outlined in that document and that will require an invigorated leadership."
Mr Kelly admitted his party had not tapped into the mood in the electorate.
He said: "The Americans call it the "big mo" .
"The 'mo' stands for both momentum and mood, two aspects of the political climate that are not with the SDLP at this time."
In an apparent criticism of party leader John Hume's election message, Mr Kelly said:
"The SDLP to a large degree tried to lead the electorate to a new type of politics, which has been loosely described as 'post-nationalist'."
"The bald reality is the mood in the nationalist electorate is not ready for that message."
Other voices within the SDLP have been more muted.
The SDLP chairman Alex Attwood said there were many levels of organisation within the party which would have to be looked at.
" It has been difficult to recruit people, politics isn't sexy when you're in the centre ground "
The party's Finance Minister in the Northern Ireland Assembly, Mark Durkan, said his party had suffered because it "put process before party".
He told BBC Northern Ireland's election programme: "Too often in councils and indeed in politics in general we allow ourselves to be taken for granted.
"We spend a lot of time attending to the problems of other parties, or problems caused by other parties, and not enough taking care of our own position."
And newly elected Craigavon councillor Dolores Kelly admitted: "It has been difficult to recruit people.
"Politics isn't sexy when you're in the centre ground.
"We obviously would need to see how we can attract young voters."