Seamus Mallon has said Sinn Fein used his former SDLP party leader John Hume to gain respect in the United States.
Former deputy first minister Mr Mallon told BBC Radio Ulster's Talkback that Hume was "no fool" but Sinn Fein leaders played him "like a 3lb trout".
Mr Hume's presence gave republicans a status almost validating 30 years of Troubles violence, he said, in an interview being broadcast today.
Mr Mallon remained critical of both the DUP and Sinn Fein, describing them as "the two biggest bullies on each side".
"What worries me most is that the two major parties don't seem to, in their hearts, believe in the whole thesis of the Good Friday Agreement," he said.
"Is the present Executive showing any benevolence, except to their own supporters and those who vote for them?"
A Sinn Fein spokesman responded: "It is unfortunate that Seamus Mallon talks of his one-time party leader in this fashion, especially given John Hume's current health.
"And in spite of the naysayers, which included Seamus Mallon and much of the Irish establishment, the courage and the vision of John Hume and Gerry Adams and others brought about the Irish peace process and an end to the conflict."