Yousaf M. Butt, a consultant with Federation of American Scientists, who last year did a lengthy analysis of EMP for The Space Review, a weekly online journal, said, “If terrorists want to do something serious, they’ll use a weapon of mass destruction — not mass disruption.” He said, “They don’t want to depend on complicated secondary effects in which the physics is not very clear.”

Mr. Gingrich’s spokesman, R. C. Hammond, did not respond to e-mails asking for comment. But the candidate, a former history professor and House speaker, has defended his characterizations as accurate. At a forum in Des Moines on Saturday for military veterans, Mr. Gingrich said an electromagnetic pulse attack was one of several pressing national security threats the United States faced. “In theory, a relatively small device over Omaha would knock out about half the electricity generated in the United States,” he told the veterans.

He also referred to the apocalyptic novel “One Second After,” written by a friend and co-author of his historical novels, William R. Forstchen. The book describes an electromagnetic pulse attack on America, conjuring a world in which cars, airplanes, cellphones and refrigerators all die, and gangs of barbarians spring to life.

In the book’s foreword, Mr. Gingrich calls the grim scenario a potential “future history” that should be “terrifying for all of us.” He says he knows its frightening plausibility from decades of personal study.

Electromagnetic pulse is a real phenomenon, though many scientists consider it yesteryear’s concern. It came to light in July 1962 when the American military detonated a hydrogen bomb high above the Pacific. In Hawaii, street lights suddenly went out.

The riddle got little direct investigation because in 1963 the superpowers agreed to end all but underground detonations of nuclear arms. But theoretical studies continued, and worries rose over the decades as electronic circuits became ever more sophisticated and fragile.

Mr. Gingrich is part of a conservative movement that calls EMP an underappreciated danger. In Congress, spurred by Representative Roscoe G. Bartlett, Republican of Maryland, members of the movement hold hearings and recommend new safeguards, especially for the nation’s power grid, for which protective steps could run into many billions of dollars.