Scientists who study the risk of nuclear war recently moved the hands of the symbolic Doomsday Clock to 2½ minutes before midnight — meaning they believe that the world is closer to nuclear catastrophe than it has been since 1953 after the United States and Soviet Union tested hydrogen bombs. The Bulletin of the Atomic Scientists, which created the clock in 1947, says that President Trump is the main reason for this worrisome development.

Mr. Trump came to office with little knowledge of the vast nuclear arsenal and the missiles, bombers and submarines it contains. He has spoken, alarmingly, about deploying this weaponry against terrorists and about expanding America’s nuclear capabilities. He has said he values unpredictability, meaning presumably that he wants to keep other nations on edge about whether he will use nuclear weapons.

“Let it be an arms race,” he told a television interviewer in December. During a debate three months earlier he contradicted himself, saying that “I would certainly not do first strike,” then adding, “I can’t take anything off the table.” What’s worrisome about all this is that it is the opposite of what Republican and Democratic presidents have long sought, which is to ensure that these weapons are not used precipitously if at all.

It is the fear of such precipitous action that has led Senator Edward Markey of Massachusetts and Representative Ted Lieu of California, both Democrats, to propose legislation to prohibit any president from launching a first-strike nuclear weapon without a declaration of war from Congress.