CHAPEL HILL, N.C., March 22 — John Edwards, the North Carolina Democrat, said Thursday that his wife’s cancer had returned in incurable form. He proclaimed that he would continue his bid for the presidency, saying, “The campaign goes on strongly.”

The announcement here by Mr. Edwards and his wife, Elizabeth, followed an emotional 72-hour stretch.

On Monday, Mrs. Edwards reported pains to her doctor and learned that her cancer might have returned. On Tuesday, Mr. Edwards cut short a trip in Iowa to fly back on a charter plane. The couple, alone, went to the University of North Carolina hospital on Wednesday for a daylong battery of tests that confirmed the diagnosis.

The tests completed, the Edwardses summoned a handful of aides to the living room of their home near here at 6 p.m. Wednesday, where Mr. Edwards informed them of the diagnosis and Mrs. Edwards said the campaign would continue unabated, participants said. The session was described as emotional, with several aides fighting back tears.

Across Chapel Hill and back in Washington, Mr. Edwards’s aides and supporters reacted to details of Mrs. Edwards’s condition — that the cancer, if incurable, was treatable — with deep relief. The news of Mr. Edwards’s cutting short his Iowa trip had created anxiety and alarm among his campaign aides and supporters familiar with her earlier battle with breast cancer.

Mr. Edwards’s aides said he had not discussed dropping out or suspending his campaign with them. At a midday news conference Thursday, he suggested that in private talks with his wife he had offered to pull out of the race, but she had insisted he stay in.

“I’m absolutely ready for this,” Mrs. Edwards, 57, said. “I mean, I don’t look sickly, I don’t feel sickly. And I’m as ready as any person can be for that.”

Mrs. Edwards’s doctor said at the news conference that she had metastatic, or Stage 4, breast cancer, meaning that it is in an advanced stage that has spread beyond the breast and lymph nodes, in her case to the bone.

According to statistics from the American Cancer Society, only 26.1 percent of patients with Stage 4 breast cancer live five years or more, but those figures are by nature outdated and do not reflect recent medical advances.

“The bottom line is her cancer is back,” Mr. Edwards said. “We are very optimistic about this because having been through some struggles together in the past, we know that the key is to keep your head up and keep moving and be strong.”

Mr. Edwards said many patients had lived many years, managing their condition in a way he likened to people with diabetes who rely on insulin treatment.

Mr. Edwards’s political associates said the decision to continue the campaign was, characteristically of the Edwards family, made by Mr. and Mrs. Edwards alone and then conveyed to his aides. After finishing the medical and political update, the Edwardses instructed staff members to arrange the Thursday news conference before heading to an event that was part of a fund-raising retreat here.

Mr. and Mrs. Edwards spent the morning calling a few friends and supporters to tell them of her illness and his plans to continue the campaign. They arrived about 20 minutes late for the news conference at the Carolina Inn, the Chapel Hill resort where the Edwardses had their wedding reception 30 years ago, and which was also the site for parts of this week’s fund-raising retreat.

Facing a wall of cameras and scores of journalists, Mr. and Mrs. Edwards — at times somber, at times emotional and at times even lighthearted —recounted what they had gone through since Mrs. Edwards went to the hospital for an X-ray of a rib she thought she had broken. It was there that doctors spotted something suspicious on another rib that, as Mr. Edwards said, turned out to be cancer.

His announcement capped a morning of intense speculation and news reports — including on the Web site Politico.com — that he would at least suspend his bid for the Democratic nomination.

Some Democrats said Mr. Edwards’s decision to continue his bid could pose some risks should his wife’s illness worsen and raise questions about his decision to continue campaigning in a race where he has often placed third in polls in an intensely competitive Democratic field.

Mr. Edwards made his priorities clear. “Any time, any place that I need to be with Elizabeth,” he said, “I will be there, period.”

Mrs. Edwards had a prominent role in his failed 2004 campaign for the president, both as a behind-the-scenes player intimately involved in the management of the campaign and on the trail on behalf of her husband. She was widely viewed as an effective surrogate for Mr. Edwards in 2004, particularly with blue-collar audiences. Since then, her struggle with cancer, chronicled in a best-selling book, “Saving Graces,” has raised her profile across the nation and made her a popular and sympathetic symbol of a woman who had survived a bout with cancer.

To a large extent, aides said, her ability to play a similar role in this campaign would depend on the success in treating the disease as well as how difficult it might be to deal with side effects of treatment.

Mr. Edwards’s two main Democratic opponents, Senators Hillary Rodham Clinton and Barack Obama, offered expressions of sympathy and support.

“Our thoughts and prayers are with Elizabeth Edwards and the whole family,” Mr. Obama said in an interview as he walked from the Capitol. “She’s a strong woman of great character and she’s a fighter.”

The White House spokesman, Tony Snow, a survivor of colon cancer, also expressed his support.

“When you have cancer, it’s very important to keep checking,” Mr. Snow said. “She’s being aggressive. She’s living an active life. And a positive attitude, prayers and people you love are always a very good addition to any kind of medicine you have.”

“So for Elizabeth Edwards, good going,” he said. “Our prayers are with you.”

Former President Bill Clinton said he watched the news conference after Mrs. Clinton called him to tell him it was on.

“Hillary called me so we could watch it,” he told reporters on his way to a fund-raiser in Manhattan. “We’ve known them for a long time, and they were our neighbors in Washington. They’ve been through so much. “

Mrs. Edwards received her original diagnosis of breast cancer at the end of the 2004 campaign, but deferred a public announcement until after the election results came in. Mr. Edwards has said he waited to announce a second bid for the presidency until the couple and her doctors were confident about her recovery.

The couple’s son Wade was killed in a car accident in 1996, a fact that several of their friends noted in discussing this latest turn of events.

Several of the current candidates for president, or their spouses, have had their own bouts with serious illness, including John McCain’s skin cancer, Rudolph W. Giuliani’s prostate cancer and Bill Clinton’s heart disease. Mitt Romney’s wife, Ann, has multiple sclerosis. In 1992, Senator Paul E. Tsongas, a Democrat running for president, said he had been cured of non-Hodgkins lymphoma. He died five years later.

The diagnosis put to a halt a busy campaign schedule for Mr. Edwards. He was leaving Nevada, Iowa, heading for a meeting with Democratic legislators in Des Moines and a meeting in Altoona when his wife called him to tell that doctors had found “something that looked suspicious in a rib” in her visit on Monday and wanted her to return on Wednesday for more tests. Mr. and Mrs. Edwards spent Wednesday at the hospital, going from test to test.

“The biopsy showed that the cancer had returned; it was malignant,” he said.

The couple sat down and told their two younger children about the change in their mother’s condition. “They’re fairly disappointed, but it doesn’t look like I’ll lose my hair with the next round of medicine,” Mrs. Edwards said at the news conference.

Harrison Hickman, Mr. Edwards’s pollster and a family friend for 10 years, said the couple had made the decision to continue the campaign on their own, and that there had been no political or strategy meetings to discuss the issue. Mr. Hickman said he had never taken a poll concerning Mrs. Edwards’s health.

“The decision was theirs,” he said. “They only called people in very late in the process to implement the decision.”

Asked for his gut sense of how the campaign would turn out for Mr. Edwards, Mr. Hickman said: “I don’t really know, I don’t. It’s another window into him, and I think it gives people a better sense of who he is and what he’s all about.”

Mr. Edwards flew to New York City on Thursday night for a fund-raiser, where he said he felt “great,” while Mrs. Edwards went with their two younger children to Boston for dinner with their daughter Cate. They planned to meet Mr. Edwards for a flight to Los Angeles for yet another fund-raiser.

Mr. Edwards is considered a top contender for the presidency. Although he has finished third in some national polls, he is leading in some early polls in Iowa and has distinguished himself in this campaign by offering a comprehensive health care coverage plan and pushing for an end to the war in Iraq.

Mr. Edwards finished second in the Iowa caucuses in 2004, but pulled out after the Super Tuesday primary in March. Senator John Kerry then chose him as his running mate.