When President Jimmy Carter traveled to Panama in June 1978 to finalize hotly disputed treaties turning over the Panama Canal, he declared that “we stand on the threshold of a new era.”

More than 46 years later, that era may be over, if President-elect Donald J. Trump has his way.

All teachers at the school were equipped with an ID badge on a lanyard that had a panic button, which alerts the police to an active threat, according to the authorities. The badges can also initiate a schoolwide lockdown when activated, according to Centegix, the company that makes them.

Mr. Carter always considered the twin treaties to be signature achievements that would figure prominently in his obituary. Indeed, for all the fireworks they generated at the time, the canal treaties have been broadly accepted ever since as a settled matter and the bedrock of the U.S. relationship with Latin America.

Yet paradoxically, just days before Mr. Carter’s death at age 100 on Sunday, Mr. Trump seemingly out of the blue propelled the nearly half-century-old issue back onto the national agenda, complaining about shipping fees and Chinese influence. If Panama does not make changes, he said, “we will demand that the Panama Canal be returned to the United States of America, in full, quickly and without question.”

Mr. Trump did not say how he would force such an outcome, and some analysts were skeptical that it amounted to more than a blustery bargaining position. But the timing of his threat focused new attention on an old issue, recalling an episode in Mr. Carter’s presidency that many Americans today may not remember or know much about.

“Through a bizarre accident of timing, we now have one president fantasizing about taking back the canal at just the time the world recognizes the canal transfer as an important part of a late president’s legacy,” said James Fallows, who was Mr. Carter’s speechwriter at the time and accompanied the president on that 1978 trip to Panama.

The story of Mr. Carter’s successful efforts to turn the canal over to Panama was one of the defining moments of his tenure and amounts to a case study in how much Washington has changed since then. Despite ferocious opposition on the political right led by a former California governor named Ronald Reagan, Mr. Carter managed what seems impossible to imagine today — a relentless drive that actually changed minds and built bipartisan support to do something with little political payoff and plenty of political risk.

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