WASHINGTON (AP)  Former presidential candidate John Edwards, who won nationwide praise and sympathy as he campaigned side by side with his cancer-stricken wife, Elizabeth, admitted in shame Friday he had had an affair with a woman who produced videos as he prepared to launch his campaign.

Acknowledging a sex scandal he had dismissed as "tabloid trash" only last month, Edwards said he had told his wife and family long ago, but "I had hoped that it would never become public."

He denied fathering a daughter, born to the woman with whom he had the affair, and offered to be tested to prove it. A former Edwards campaign staff member professes to be the father.

The former North Carolina senator, who was the Democratic vice presidential nominee in 2004, confessed to ABC News that he had lied repeatedly about the affair with 42-year-old Rielle Hunter. Hunter's daughter, Frances Quinn Hunter, was born on Feb. 27 this year, and no father's name is given on the birth certificate filed in California.

After the story broke Friday, Edwards released a statement that said, "In 2006, I made a serious error in judgment and conducted myself in a way that was disloyal to my family and to my core beliefs. I recognized my mistake, and I told my wife that I had a liaison with another woman, and I asked for her forgiveness. Although I was honest in every painful detail with my family, I did not tell the public."

"I was and am ashamed of my conduct and choices," he said. "With my family, I took responsibility for my actions in 2006, and today I take full responsibility publicly."

Edwards declared his presidential candidacy in December 2006. His wife was at his side that day and campaigned enthusiastically with him and by herself in the months that followed. She announced in March 2007 that her cancer, formerly in remission, had returned and there apparently was no cure.

She and her husband said it was important for the campaign to continue.

Edwards dropped out midway through this year's primaries after it became apparent he could not keep up with front-runners Barack Obama and Hillary Rodham Clinton. He recently endorsed Obama and has been mentioned as a possible running mate.