Several months ago, John Stansbury, a soft-spoken professor from Omaha, Neb., took his 12-year-old grandson to a public meeting to discuss Keystone XL, the proposed mega-pipeline that would carry oil from Canada across his home state to the Gulf of Mexico. At the time, Stansbury knew almost nothing about the pipeline and had never done anything particularly political. “I'm not really an activist,” he says, a bit sheepishly. But he wanted his grandson to “see democracy at work.”

The 61-year-old civil engineer also happens to be an expert in the transport of hazardous materials. And as he learned more about Keystone XL, he saw a disaster in the making. After the meeting, Stansbury began poring over official risk assessments of the pipeline and thought they grossly underestimated the probability of a spill. He was so troubled that he did something he’s never done before—he courted media attention. He drafted an independent report on the pipeline, asked the organization Friends of the Earth to help announce his findings, and held a press conference. He predicts the pipeline could have approximately 91 significant spills over the next 50 years—eight times as many as the energy company TransCanada estimated.

Keystone XL has enough strikes against it to turn political neophytes like Stansbury into first-time activists, and to compel some unusual alliances between conservatives and progressives. ...