To McCain, the enmity was nothing new. Though he is a decorated Navy flier who languished nearly six years in North Vietnamese prisons, some of the roughly 5,000 people who make up the POW/MIA movement consider him a bitter enemy. The senator has harshly criticized some professional POW/MIA groups for promoting conspiracy theories that American soldiers are still being held captive. At a 1992 Senate hearing McCain tore into an activist with such vehemence that the woman broke down in tears.

A handful of POW/MIA extremists are now retaliating with a noisy attempt to derail his presidential campaign. On Web sites and in faxes and e-mails to reporters and political operatives, they charge McCain with leading a government conspiracy to cover up the truth about missing soldiers–and accuse him of trying to keep official records secret to hide evidence that he collaborated with his communist captors. McCain ally Orson Swindle, a Federal Trade commissioner and former POW, says he was warned months ago that opponents planned to start a whispering campaign alleging McCain’s time in captivity left him mentally unstable. Other McCain enemies are less subtle. Ted Sampley, an ex-Green Beret who runs a Web site denouncing McCain as “the Manchurian Candidate,” says he’s distributing his material to the senator’s rivals. “Hell, we’re not whispering,” he says. “We’ve been shouting this stuff.”

That McCain, of all people, came to be the target of such wrath is “an irony almost beyond belief,” says Pennsylvania Gov. Tom Ridge, a fellow Vietnam War vet. McCain had returned to the United States in 1973 an angry man after years of imprisonment and torture. According to a never released Pentagon report reviewed by NEWSWEEK, McCain’s debriefers asked him about antiwar activists like Jane Fonda, who had traveled to Hanoi and proclaimed that POWs were well treated. McCain said he felt “hatred” for them and feared “becoming violent” if he met them.

When McCain first ran for Congress in 1982, families of missing soldiers embraced him. They assumed McCain would take a tough stand against Hanoi. But by then, McCain’s anger had begun to mellow. In 1985, the 10th anniversary of the fall of Saigon, he returned to Vietnam. He says the time had come for “reconciliation and healing.” Some MIA families were outraged. “There was a feeling of betrayal,” says Ann Mills Griffiths, president of the National League of Families, the largest MIA group.

The friction only worsened in 1992, when the Senate held hearings to investigate the possibility that American soldiers were still alive in the jungles of Southeast Asia. McCain was offended that MIA activists were holding out false hope to suffering families–and he let his irritation show. “So much of this was pure fantasy,” he says. The panel concluded there was no credible evidence of living MIAs.

Then came what his enemies regard as the ultimate betrayal. In 1995 McCain supported Bill Clinton’s efforts to normalize diplomatic relations with Vietnam. The MIA activists concluded there was only one explanation for the senator’s apostasy: the Vietnamese had “turned” him. When McCain recently backed a Pentagon request to keep POWs’ debriefing papers secret to protect the privacy of former prisoners, MIA activists took it as further evidence that he was covering up his traitorous behavior. In fact, the papers contain nothing incriminating. McCain’s debriefing report graphically supports the story of a defiant prisoner trying to withstand brutal torture.

McCain doesn’t seem to worry that the conspiracy theories will do any real damage to his status as a war hero–or as a candidate. Even so, the nastiness of the smear campaign has tested McCain’s legendary temper–a trait he has been trying to play down as a candidate. These days, he tries not to pay it much mind. “Some of this is so hurtful that you can’t even read it,” he told NEWSWEEK. McCain is determined to get past the hard feelings. He once told Clinton that he had grown weary of “looking back in anger.” His foes, however, haven’t tired at all.