COOPERSTOWN, N.Y. — The letter that arrived in Kim Jastremski’s mailbox on County Highway 52 suggested that she stop protesting the possibility of natural gas drilling. It seemed more of a threat than a request.

Computer-generated, unsigned and sent to about 10 other opponents of a practice known as fracking, it compared them to Nazis and said they were being watched while picking up their children at school in their minivans.

Jennifer Huntington’s abuse is more public, like comments online suggesting that people find out where her dairy sells its milk so that they can stop buying it, or the warning that her farm, which has a lease with a gas company, “will fall like a house of cards when your water is poisoned.” She and other drilling proponents have also been called “sellout landowners that prostitute themselves for money.”

The debate over horizontal hydraulic fracturing, or fracking, the injection of huge quantities of chemically treated water underground to free up natural gas, has become increasingly contentious across the Eastern United States, with dozens of communities passing or considering bans. But that ill will often takes its most intimate form in small towns and rural areas like this one, best known as the home of baseball’s Hall of Fame, where fracking has emerged as the defining, non-negotiable political issue.