Nearly 36 years ago, John Mellencamp stood among 10,000 family farmers and their supporters in Chillicothe, Mo., his shaggy hair flying in the hot May wind. “I’m not here against any specific person,” he told the crowd, and television cameras, before performing a three-song set. “I hope for a moment we can have your voices be the voices of millions of people across the United States who are suffering the exact same thing.”