Republicans were not as harsh nor so Trump-centric. But Senator Bob Corker, the Tennessee Republican who is the committee’s chairman, and recently expressed concern that Mr. Trump could lead the country to World War III, said it was important to examine the “realities of this system” by which the use of nuclear weapons is decided. He’s right.

Mr. Trump has brought on himself this examination of his authority to order the launch of the world’s most deadly weapons. His erratic, taunting threats to rain “fire and fury” on North Korea and even destroy the country, his glib talk about nuclear weapons and his impulsiveness generally raise serious questions about his willingness to incite war.

He is engaged in a dangerous game of chicken with Kim Jong-un, the North Korean leader, who has kept up his own steady stream of bombastic insults against Mr. Trump and threatened attacks on the United States with an arsenal that has gone from zero to at least 20 nuclear weapons, plus the missiles to deliver them, over the past 30 years.

The president’s sole control of nuclear launches stems from the Atomic Energy Act of 1946, passed when there was more concern about hawkish generals than elected civilian leaders. C. Robert Kehler, a retired Air Force general who once headed the Strategic Command that oversees the nuclear arsenal, said at the hearing on Tuesday that the military could refuse to follow what it considers a disproportionate and unnecessary order. He said he did not know what the president’s response would be in such a case. But Brian McKeon, a former Pentagon official, told the committee that the president could appoint a new general and defense secretary to carry out his orders — further evidence, not at all reassuring, of the president’s unilateral powers.

Senator Ed Markey of Massachusetts and Representative Ted Lieu of California, both Democrats, have introduced legislation to bar the president from launching a first nuclear strike without a declaration of war by Congress. A president would, of course, still have the power to retaliate if America was attacked, but their bill could help restrain a trigger-happy president. Another idea would be to stipulate that the vice president or the secretaries of state and defense, or all three, must concur in any decision to strike first with nuclear weapons.