SDLP leader Margaret Ritchie is to face a leadership contest at the party's annual conference in November.
Deputy leader and MLA for Mid-Ulster, Patsy McGlone, has thrown his hat into the ring after weeks of speculation.
The move comes after growing frustration in the party after May's Assembly elections in which the SDLP lost two seats. the outcome has been blamed on weak leadership and poor organization of the party machine.
Ritchie has been in the leadership post for less than two years.
Announcing his intention to stand. Mr. McGlone said he had been encouraged to do so by friends and colleagues who shared his belief that "now is the time for change."
"I am proud of my Irishness and proud to offer myself as leader of a party which represents an Irishness of inclusivity, regardless of race, color or creed.
"These core values, articulated by John Hume and Seamus Mallon, are precisely what attracted me to the SDLP in the first place. The SDLP has revolutionized Irish politics. The founding principles of the party are as relevant today as they were during the civil rights campaign," McGlone said.
And he contibued: "The unity of the people of Ireland, a unity built on trust, along with practical social democracy - being there for people through the good and bad times - lie at the heart of every SDLP member and supporter.
"The way forward for us needs three strategic elements: strong leadership, clarity of message and radically improved grassroots organization. " Ritchie, meanwhile, said she would contest any leadership challenge.
"Anyone in the parliamentary assembly group is free to put their name forward if they get the nominations," she said.
"But at the end of the day, I am the leader."
Ritchie replaced Mark Durkan as leader at the start of 2010 after defeating the South Belfast MP Alasdair McDonnell by 35 votes. McDonnell has been linked in reports in recent days to a leadership bid of his own.
In the 2010 Westminster election, the party held its three seats and Ritchie was elected MP for South Down. However, this year's election outcome, and her performance during television debates, has seen her come in for criticism from within and without the party.
Discontent grew further in certain sections of the party when Ritchie overlooked her deputy McGlone for the environment minister's job at Stormont, giving the post instead to close ally, Alex Attwood.
