Former presidential candidate John Edwards, who won nationwide praise and sympathy as he campaigned side-by-side with his cancer-stricken wife, Elizabeth, admitted Friday he had had an extramarital affair with a woman who produced videos for his campaign.

'Although I was honest in every painful detail with my family, I did not tell the public.'— John Edwards

Acknowledging a sex scandal he had dismissed as "tabloid trash" only last month, Edwards said he had confessed to 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 read: "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.

"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.

Recently endorsed Obama

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.

He was John Kerry's running mate in 2004 when Kerry lost to President George W. Bush.

In his statement, he said, "It is inadequate to say to the people who believed in me that I am sorry, as it is inadequate to say to the people who love me that I am sorry.

"In the course of several campaigns, I started to believe that I was special and became increasingly egocentric and narcissistic. If you want to beat me up, feel free. You cannot beat me up more than I have already beaten up myself. I have been stripped bare and will now work with everything I have to help my family and others who need my help."

The National Enquirer first reported on the affair in October 2007, in the run-up to the Democratic primaries, and Edwards denied it.

"The story is false," he told reporters then. "It's completely untrue, ridiculous." He professed his love for his wife, who had an incurable form of cancer, saying, "I've been in love with the same woman for 30-plus years and as anybody who's been around us knows, she's an extraordinary human being, warm, loving, beautiful, sexy and as good a person as I have ever known.

So the story's just false."

National Enquirer proven to be right

Last month, the Enquirer carried another story stating that its reporters had accosted Edwards in a Los Angeles hotel where he had met with Hunter after her child's birth. Edwards called it "tabloid trash," but he avoided reporters' inquiries, as did his former top aides.

In an interview, scheduled to air on the ABC News program Nightline Friday night, Edwards said the tabloid was correct when it reported on his meeting with Hunter at the Beverly Hills Hilton last month.

A number of mainstream news organizations had looked into the adultery allegations but had not published or aired stories. But newspapers in Charlotte and Raleigh, N.C., recounted the Enquirer's allegations in prominent articles on Thursday.

The Edwardses have three children — Cate, Jack and Emma Claire.

Another son, Wade, died at age 16 in a 1996 car accident.

Campaign manager says he's angry

David Bonior, Edwards' campaign manager for his 2008 presidential bid, said he was disappointed and angry at Friday's news.

"Thousands of friends of the senator's and his supporters have put their faith and confidence in him, and he's let them down," said Bonior, a former congressman from Michigan. "They've been betrayed by his action."

Asked whether the affair would damage Edwards' future aspirations in public service, Bonior replied: "You can't lie in politics and expect to have people's confidence."

In 1999, when Edwards was a senator, he said of then-president Bill Clinton and his affair with Monica Lewinsky: "I think this president has shown a remarkable disrespect for his office, for the moral dimensions of leadership, for his friends, for his wife, for his precious daughter. It is breathtaking to me the level to which that disrespect has risen."

In 2006, Edwards' political action committee paid $100,000 in a four-month span to a newly formed firm run by Hunter, who directed the production of four web videos showing Edwards in supposedly candid moments, as well as in a public speech talking about morality.