Fianna Fail TD Eamon O Cuiv has warned the party founded by his iconic grandfather is facing "demise" and has said there was an "absolute collapse in self-belief" in the party.
Alternative to demise, Mr O Cuiv told the Sunday Independent Fianna Fail will become a "small niche party".
Mr O Cuiv, known as Dev Og, is grandson of Eamon de Valera, founding leader of Fianna Fail who served multiple terms as head of government and head of state.
The leadership of Micheal Martin will be in sharp focus at the Fianna Fail Ard Fheis next month.
An attempt to oust him may be made after the Carlow-Kilkenny by-election in May if the party fails to win. Yesterday, however, Mr O Cuiv said: "The danger if you decide he is gone is that you replace Micheal with another Micheal."
In a move that will intensify pressure on Mr Martin, Mr O Cuiv said the party urgently needs to improve its standing with the public.
A Red C new poll last night had Fianna Fail again failing to break past the 20pc mark, with support for the party on just 18pc.
"If we continue to poll at 18pc, this will see the demise of that Fianna Fail that people know. Fianna Fail will instead become a small niche party like the SDLP."
In a hard-hitting analysis, the former deputy leader, who contested the leadership four years ago, added: "There was more fight in Fianna Fail in 2011 and 2012 than there is today. A great lassitude has spread across the party from top to bottom. There has been an absolute collapse in self-belief. We are becalmed. If all you see in Fianna Fail is nostalgia and lethargy it will very quickly be all over for Fianna Fail. If our party membership thinks our future is as a new SDLP they will fade away very quickly".
Mr O Cuiv said: "There is a difference between the top of Fianna Fail and the supporters that is utterly corrosive." He said: "When even the membership wonder why would you vote for Fianna Fail you have a problem. You rely in politics on absolute belief and absolute enthusiasm, that's not there anymore."
Mr O Cuiv believed that on current trends Fianna Fail was looking at becoming a "25 to 35 seat party - that would be, at best, the third largest party."
His warning will increase concerns that, under Mr Martin's leadership, Fianna Fail face being junior coalition partners to FG or Sinn Fein.
In an indication of the growing concerns within the party, the Sunday Independent has learned that an elaborate plan to put up a candidate to oppose Micheal Martin for the post of President of Fianna Fail at the Ard Fheis, had been hatched. But the unprecedented challenge to the leader was aborted owing to the difficulty of securing a suitable 'stalking horse' for the post.
The Sunday Independent understands that after another dismal opinion poll last week, a Fianna Fail cumann has put forward a motion demanding Mr Martin be replaced as party leader by Mr John McGuinness, the Chairman of the Public Accounts Committee.
Mr O Cuiv said:"We need a very radical plan to be unveiled over the coming months or it will be too late. This is not a time for caution; fortune favours the brave".