SDLP leadership challenger Patsy McGlone yesterday warned the party's message became "muddled" in recent years.
It is the first direct attack on the style and approach of current leader Margaret Ritchie, but the internal competition is expected to increase in coming weeks.
There have been indications in the last seven days that Ms Ritchie has been under pressure to stand down to prevent further fragmentation within the party.
Deputy leader Mr McGlone said: "The SDLP has in recent years failed to communicate effectively. The message has become muddled and misunderstood. That must change."
The Mid-Ulster MLA said since the May Assembly elections in which the SDLP lost two seats a canvass of opinion has showed a "clear consensus" that immediate change is "absolutely vital".
Writing in the News Letter, Mr McGlone paraphrased the poet John Hewitt saying: "I am proud of my Derry roots, my Ulster identity, and - like many Protestants and unionists I know - to call myself Irish. We have more in common than that which divides us."
But he said his aim if elected leader will be to convince people holding opposing cultural and constitutional views "that our shared problems north and south are usually best faced together".
Ms Ritchie, who took over from Mark Durkan just over 18 months ago, has clearly indicated she intends to fight the challenge.