WASHINGTON — Elizabeth Edwards is emerging as a singular force in the Democratic presidential primary, displaying a candor often unseen in her candidate husband and, analysts say, harnessing the outpouring of sympathy she has gained from a public battle with cancer.

She is speaking out more and more on policy matters such as health care and gay rights, and her role in John Edwards’s campaign may in some respects rival President Clinton’s: They both are close advisers and surrogates whose popularity likely exceeds that of their spouse.

A day after Mrs. Edwards criticized Senator Clinton’s record on women’s issues, she made her solo debut in a television ad, touting Mr. Edwards’s “toughness” in a 30-second spot. The ad will run in New Hampshire, an early primary state where the former North Carolina senator is ramping up his efforts as he slips in the polls. The most recent survey had him falling to fourth place from third among the declared candidates for the first time, behind Governor Richardson of New Mexico. In recent weeks, the distinctions between husband and wife have become clear.

While he is prone to taking veiled swipes at Mrs. Clinton and Senator Obama of Illinois, Mr. Edwards has appeared reluctant to criticize his opponents directly — at a Democratic debate last month, he tried to contrast his position on the war with theirs but sheepishly mentioned them by name only after prodding by the moderators. That is not the case with Mrs. Edwards, who, in an interview published Tuesday on Salon.com questioned Mrs. Clinton’s qualifications as an advocate for women, criticized her health care plan, and said the former first lady’s oft-repeated stance on abortion — that it be “safe, legal, and rare” — made her “uncomfortable.”

Political analysts say using Mrs. Edwards to speak for her husband on women’s issues is natural and expected, particularly with a woman also in the race.

But her public image is indisputably linked to her well-publicized struggle with breast cancer. Mrs. Edwards, 58, was first diagnosed shortly before the 2004 presidential election, and in March of this year, she and Mr. Edwards disclosed that the cancer had returned. At that time, she said it had spread to a rib and that while it was treatable, it could not be cured.

The announcement generated widespread sympathy for the Edwardses and even led to a modest boost in his standing in the polls. But four months later, as Mr. Edwards has struggled through scrutiny of his wealth and jokes about $400 haircuts, Mrs. Edwards’s popularity has remained high.

“She’s the best thing he’s got going,” the director of the University of Virginia’s Center for Politics, Larry Sabato, said.

Fair or not, public concern for Mrs. Edwards’s health may provide a political advantage for the campaign. “She gives him cover,” Mr. Sabato said. “Because of the cancer factor, none of the other candidates can respond.”

Yet that is not Mrs. Edwards’s sole contribution to her husband’s bid, the director of the Marist Institute for Public Opinion, Lee Miringoff, said. “She has good credibility and good standing with the public,” he said. “Independent of her health problems, she has an appeal with voters.”

Mrs. Edwards is said to be a key adviser for the campaign. She often addresses policy issues, and not always with the same position as her husband. The most notable difference is on gay marriage; she supports it, while he has gone only so far as to endorse civil unions. In seeking to capitalize on Mrs. Edwards’s appeal, the risk for the Edwards campaign mirrors the concerns the Clintons may have about the former president’s role — they do not want a popular spouse to upstage the candidate. Advisers to Mr. Edwards appear all too aware of that danger. In a conference call yesterday to announce the ad featuring Mrs. Edwards, top aides praised her as an effective spokeswoman and surrogate for her husband, but they were careful not to overstate her prominence in the campaign.

Asked whether Mrs. Edwards had become a more “dominant” voice than Mr. Edwards, the deputy campaign manager, Jonathan Prince, scoffed. “I think that’s silly,” he said. “I think John Edwards is clearly the most dominant voice in this campaign.”

They also downplayed the significance of Mrs. Edwards’s recent statements about Mrs. Clinton, rejecting the notion that she is assuming the role of chief mudslinger. “Elizabeth’s not going after anybody,” Mr. Prince said. “Elizabeth’s going out there and advocating on behalf of her husband.”

A senior adviser, Joseph Trippi, was similarly dismissive: “Shock, surprise, breaking news. Elizabeth Edwards supports her husband,” he said. “Elizabeth is Elizabeth. She’s a strong, effective advocate, and that’s what she’s doing.”