FALL RIVER, Mass. — In May 2013, two environmental activists piloted a lobster boat close to the vast pile of coal at the Brayton Point Power Station, the hulking plant that overlooks this industrial inlet near the border between Massachusetts and Rhode Island.

With the tiny boat, the two men — Jay O’Hara, 32, and Ken Ward, 57 — dropped anchor and for a day blocked a freighter with a 40,000-ton shipment of coal, and they were charged with conspiracy, disturbing the peace and other violations.

The two were scheduled to be tried on Monday, and they planned to deploy an old legal argument called the necessity defense: They had no choice but to act because the consequences of climate change are so dire. But instead of a jury trial, the major charges were dropped or downgraded by the district attorney, who said, in effect, that he was sympathetic to the defendants’ point of view on climate change.

Sam Sutter, the Bristol County district attorney who dropped the conspiracy charge and downgraded the others to civil infractions, strode out of the Fall River Justice Center clutching an article on climate change written by the environmentalist and author Bill McKibben, who was prepared to testify as an expert witness for the defense. Mr. Sutter told more than 100 climate activists who had gathered outside that he had reached his decision in part because of environmental concerns.