A former aide to John Edwards, the one-time senator whose presidential campaign last year came spectacularly to grief on the rocks of an illicit affair, is set to claim in a tell-all book that his erstwhile boss assured him he would be "taken care of for life" if he pretended to be the father of a child the woman bore.

The remarkable assertion is one of the highlights of a proposal for his book reportedly presented to the St Martin's Press publishing house. In it, the aide, Andrew Young, also claims that Mr Edwards made a sex videotape with his former mistress, Rielle Hunter, which he subsequently discovered.

Asking Mr Young to pose as the father, Mr Edwards purportedly told him, "You know how much I love you. You know I'd walk off a cliff for you, and I know you'd walk off a cliff for me." The candidate, who was once the Democratic Party's golden boy and was John Kerry's running mate in 2004, went on: "I will never forget this. And I will always be there for you."

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The book will apparently assert that the father of the child is indeed Mr Edwards. The one-time rival of Mr Obama in the 2008 primaries has so far not submitted to a DNA test to determine the truth of this. However, he is still at the heart of a federal investigation into whether campaign money was used in connection with the affair or to help with the efforts to cover it up. Mr Young says that the FBI has already interviewed him in this regard.

The startling new details contained in the proposal – and the prospect of the book coming out – make the chances of a political rehabilitation for Mr Edwards seem more remote than ever. "In a culture in which it is almost impossible to sin yourself into permanent banishment... Edwards may have pulled the feat off," Jay Bookman, a commentator for the Atlanta Journal-Constitution noted yesterday. "It is impossible to feel sympathy for the man."

Not that Mr Young himself is likely to solicit very much sympathy. As part of his alleged deal with Mr Edwards, he invited the pregnant Ms Hunter to move in with him, his wife and their three children. This most peculiar co-habitation ended only after Ms Hunter gave birth.

The saga is thus also about the suffering of the wives involved. In her own recent book, Resilience, Elizabeth Edwards admits that there is one "pathetic" former aide to her husband she cannot bring herself even to name. "I will call him Jim," she writes, calling him an "obsessed fan", who showed an "unbridled loyalty" to her husband and a "willingness to do anything". Few have any doubts that she is referring to the aide who has now turned author, Mr Young.