Three months after Elizabeth Edwards said that her cancer had returned in inoperable form, her role and influence in John Edwards’s presidential campaign is undiminished. She has made a flurry of charged public appearances, become a regular presence advising Mr. Edwards on the campaign trail, and wields behind-the-scenes influence in many internal campaign decisions, aides said. Mrs. Edwards has also become a free operator on behalf of her husband of 29 years, a development that her friends suggest reflects the clarity and perspective that come from her cancer diagnosis, and her increasingly confident political instincts as she advises Mr. Edwards,...