Losing the G.O.P. nomination in 2000 gave McCain time to catch and treat the cancer at an early stage, which possibly saved his life. "If it was left alone, the risk was high that that melanoma would not just have become thicker but would also almost certainly have spread to the lymph nodes," says Dr. Jeffrey Lee, a cancer physician at the M.D. Anderson Cancer Center, who did not participate in McCain's care. "And the assumption would be that could occur within a period of a few months, if it hadn't happened already."

Eight years later, McCain at 71 finds himself on his way to another Republican Convention, and the questions about his health are no longer secondary to his political fortunes. If he were to win in November, he would become the oldest first-term President in U.S. history. To make the issue more pronounced, his likely opponent is young enough to be his son; at 46, Barack Obama hopes to become the fifth youngest President ever.