WASHINGTON — 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, Mr. 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. Ms. 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.

In a statement provided to The Associated Press, Elizabeth Edwards confirmed that her husband had confessed the extramarital affair to her in 2006 but that after a “long and painful process” she was now standing by him.

After the story broke Friday, Mr. 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.”

Mr. Edwards declared his presidential candidacy in December 2006. His wife 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.

Mr. 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 Mr. Obama and has been mentioned as a possible running mate.

He was John Kerry’s running mate in 2004 when Senator Kerry lost to President 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.”

In her statement Friday, Elizabeth Edwards said it wasn’t easy to find out about the extramarital affair in 2006. She called the affair a “terrible mistake” but said the healing process was “oddly made somewhat easier” after her diagnosis of breast cancer in March 2007.

“This was our private matter, and I frankly wanted it to be private because as painful as it was I did not want to have to play it out on a public stage as well,” she said. “Because of a recent string of hurtful and absurd lies in a tabloid publication, because of a picture falsely suggesting that John was spending time with a child it wrongly alleged he had fathered outside our marriage, our private matter could no longer be wholly private.”

Mrs. Edwards said her family has been through a lot and pleaded for privacy.

“I am proud of the courage John showed by his honesty in the face of shame,” she said. “I ask that the public, who expressed concern about the harm John’s conduct has done to us, think also about the real harm that the present voyeurism does and give me and my family the privacy we need at this time.”

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

Last month, the Enquirer carried another story — the blaring headline referred to an Edwards “love child” — stating that its reporters had accosted Mr. Edwards in a Los Angeles hotel where he had met with Ms. Hunter after her child’s birth. Mr. Edwards called it “tabloid trash,” but he generally avoided reporters’ inquiries, as did his former top aides.

He said in his statement Friday he had “used the fact that the story contained many falsities to deny it,” and he called that “being 99 percent honest.”

In an interview, scheduled to air on ABC News’ “Nightline” Friday night, Mr. Edwards said the tabloid was correct when it reported on his meeting with Ms. 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 16 in a 1996 car accident.

David Bonior, Mr. 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 Mr. Bonior, a former congressman from Michigan. “They’ve been betrayed by his action.”

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

CBS News chief White House correspondent Bob Schieffer spoke to John and Elizabeth Edwards on Friday afternoon. Mr. Schieffer reported that when Mrs. Edwards got on the phone, “she was obviously in tears” and that she said “this is really, really tough,” but she confirmed her husband told her about the affair in 2006 and the couple had decided to “move on.” When Mr. Schieffer asked Mr. Edwards how his wife was doing, he said “she is just amazing, like she always is.”

In his statement, Mr. Edwards denied making any payments to Ms. Hunter or “or to the apparent father of the baby.” Late Friday, Dallas-based attorney Fred Baron, former national finance chairman for Mr. Edwards, said in a statement he decided on his own to “help two friends and former colleagues rebuild their lives when harassment by supermarket tabloids made it impossible for them to move forward on their own.”

Mr. Baron, who did not return messages left by The Associated Press, didn’t mention anyone by name in his statement. He said the assistance was offered and accepted without the knowledge of Mr. Edwards or anyone else.

In 1999, when Mr. Edwards was a senator, he said of President 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, Mr. Edwards’ political action committee paid $100,000 in a four-month span to a newly formed firm run by Ms. Hunter, who directed the production of four Web videos showing Mr. Edwards in supposedly candid moments as well as in a public speech talking about morality.

The payments from Edwards’ One America Committee to Midline Groove Productions LLC started on July 5, 2006, five days after Ms. Hunter incorporated the firm in Delaware.

Midline provided “Website/Internet services,” according to reports that Mr. Edwards’s PAC filed with the Federal Election Commission.

Midline’s work product consists of four YouTube videos showing Mr. Edwards in informal settings as he prepares to make speeches in Storm Lake, Iowa, and Pittsburgh, as he prepares for an appearance on “The Daily Show With Jon Stewart” and travels in Uganda in 2006.

Mr. Edwards’s PAC followed the six-figure payment with two smaller payments totaling $14,461, the last on April 1, 2007.

At the time, Ms. Hunter was compiling the videos in 2006, Mr. Edwards was preparing his run for president.

Episode One of the four videos shows a conversation between Mr. Edwards and an unseen woman as the two chat aboard a plane about an upcoming speech in Storm Lake, Iowa.

Cutting between clips of the speech and the conversation with the woman, Mr. Edwards touches on his standard political themes, declaring that government must do a better job of addressing the great issues of the day, from poverty and education to jobs and the war in Iraq.

“I want to see our party lead on the great moral issues — yes, me a Democrat using that word — the great moral issues that face our country,” Mr. Edwards tells the crowd. “If we want to live in a moral, honest just America and if we want to live in a moral and just world, we can’t wait for somebody else to do it. We have to do it.”