"Two years ago, I made a very serious mistake, a mistake that I am responsible for and no one else," Edwards said in the interview. "In 2006, I told Elizabeth about the mistake, asked her for her forgiveness, asked God for his forgiveness. And we have kept this within our family since that time."
Edwards, 56, said he told his entire family about the affair after it ended in 2006, and that his wife, Elizabeth, who has incurable breast cancer, was "furious" but that their marriage would survive. The couple have three children, Cate, 27; Emma, 11; and Jack, 9.
Months after the confession, Elizabeth Edwards wrote a book, "Resilience," in which she discussed her husband's affair. While promoting the book on "The Oprah Winfrey Show" in May, she said no paternity test had yet taken place.
But in the summer of 2009, the National Enquirer published reports that Edwards had taken a paternity test, and the results confirmed he was the father.
Hunter gave birth to daughter Frances Quinn in February 2008, a month after Edwards ended his bid for the presidency. She listed no name for the father on the birth certificate, which created further speculation about who it might be.
Andrew Young, a close political aide to Edwards, originally claimed he was the father of Hunter's daughter, not Edwards. But Young -- who has two children with his wife, Cheri -- has since retracted that story and was expected to reveal more about the affair in the exclusive "20/20" interview Friday Jan. 29.
Soon after she was told of the affair, Elizabeth Edwards posted a message on her blog, saying that she and her children will stand by Edwards.
"Our family has been through a lot. Some caused by nature, some caused by human weakness, and some -- most recently -- caused by the desire for sensationalism and profit without any regard for the human consequences," she wrote on her blog. "None of these has been easy. But we have stood with one another through them all. Although John believes he should stand alone and take the consequences of his action now, when the door closes behind him, he has his family waiting for him."
She too, denied in the blog post that Hunter's child was fathered by her husband.
Edwards told ABC News in 2008 that he'd met secretly with former lover Rielle Hunter as recently as last month in a California hotel room at her request because "she was having some trouble, she just wanted to talk."
"I wanted her not to tell the public what had happened," he said at another point. "Very simple. That's the reason I went."
Edwards contacted ABC News about the August 2008 interview and requested that he be interviewed about the allegations. He agreed before the interview to place no limitations on the questions he would be asked, but he limited the amount of time he would be questioned. He declined to clarify exactly when the relationship began or ended but said it was over before he announced his campaign for the presidency Dec. 28, 2006.
"I think my family is entitled to every detail," Edwards said then. "They've been told every detail. … I think beyond the basics … I think that's where it stops in terms of the public because I think everything else is within my family and those privacy boundaries out to be respected."