Socialist Party TD Joe Higgins says Sinn Fein's attack on his by-election candidate shows how worried the party is about winning the seat.
Sinn Fein has admitted a party member circulated a fake message about former Socialist MEP Paul Murphy, who is running in the Dublin South-West by-election for the Anti Austerity Alliance (AAA).
Sinn Fein's candidate Cathal King is the overwhelming favourite from the start of the race, but Mr Murphy is viewed as a dark horse who has been making a late charge.
Sources say party polling conducted in recent weeks show Mr King on 25pc, but Mr Murphy on 21pc.
Sinn Fein vice-president Mary-Lou McDonald has embarrassingly been forced to repeatedly apologise to Mr Murphy after she and other senior party figures shared the fake Facebook conversation allegedly showing Mr Murphy admitting to lying about Sinn Fein policy on water charges.
But Mr Higgins says the Sinn Fein attacks are highly significant and no accident.
"The fact that leading members of Sinn Fein would propagate through social media what was palpably a lie about Paul Murphy in relation to water charges shows, I think, a huge nervousness about the support the AAA, Paul and the idea of building a mass campaign against water charges," he said.
"That, I think, explains it. Sinn Fein, I think, have been forced to change position over the course of this campaign."
Mr Murphy has been strong in pinning Sinn Fein down on a pledge to reverse the water charges. He has been calling for a boycott of water charges.
The social media image made the message appear to come from Mr Murphy's personal account. Mr Murphy accused Sinn Fein of engaging in "dirty tricks" and the party admits the message was fake.
Sinn Fein says a party member began circulating the message but he, too, was duped by the fake image.
Ms McDonald says she has "apologised sincerely" to Mr Murphy. She said she does not know the member who circulated the message but confirmed his membership of Sinn Fein.