The closely watched debate, organized and chaired by a prominent Republican Trump critic, addressed a hypothetical presidential decision to launch a nuclear first strike against an adversary.

"Let's just recognize the exceptional nature of this moment," Senate Democrat Chris Murphy said during the hearing of the chamber's Foreign Relations Committee.

"We are concerned that the president of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic, that he might order a nuclear weapon strike that is wildly out of step with US national security interests," he added.

In the event of an ongoing or imminent nuclear attack, senators and expert witnesses agreed that the president had full authority to defend the nation in accordance with the US Constitution.

Only the president can give the order to pull the nuclear trigger.

"Once that order is given and verified, there is no way to revoke it," said the committee's chairman, Senator Bob Corker, who described the hearing as the first since 1976 to focus on presidential authority over nuclear weapons.

Corker has broken publicly with Trump, warning last month that the president was setting the nation "on the path to World War III" with his statements about North Korea and verbal jousting with Kim.

In August, Trump issued an apocalyptic threat to unleash "fire and fury" on North Korea and he took to calling Kim "Rocket Man" after a series of provocative missile tests.

Kim, no stranger to verbal jousting, branded Trump a "dotard."

During Trump's 12-day trip to Asia, where Pyongyang's nuclear program was frequently discussed, Trump mocked the 33-year-old leader as "short and fat" and warned that misjudging American resolve as weakness would be "a fatal miscalculation."

The escalating war of words has alarmed US lawmakers.

'Human factor' 

But while some senators, including Democrat Edward Markey, expressed fear that in the age of Trump, an impulsive commander in chief has the power to unilaterally unleash a nuclear fusillade, the experts cautioned against legislative alterations that would broaden nuclear command authority to lower echelons. 

"I think if we were to change the decision-making process in some way to -- because of a distrust of this president, I think that would be an unfortunate precedent," testified Brian McKeon, a former undersecretary of defense under Barack Obama. 

One issue under debate was the concept of imminent threat, when the president believes a country poses a sufficient immediate danger for the US to order a pre-emptive nuclear strike.

The three experts agreed there was no strict definition of "imminent," suggesting that a missile on a North Korean launchpad might qualify but that other scenarios might be less clear.

Robert Kehler, who headed US Strategic Command from 2011 to 2013, referred to a basic military precept: "The military is obligated to follow legal orders, but is not obligated to follow illegal orders."

So, what constitutes a legal order? Kehler, a retired US Air Force general, said the military principles of "necessity" and "proportionality" also apply to decisions about nuclear weapons.

But when asked what he would do if he determined that a presidential nuclear order was illegal, Kehler hesitated about such a hypothetical.

"I don't know exactly," he responded. "The human factor kicks in."

In such a situation, McKeon said, the president could replace the commander in question, or even the secretary of defense.

"But you'd have a real constitutional crisis on your hands," McKeon said. 

The discomfort among some Republican senators was visible.

"Our adversaries are watching," said Senator Marco Rubio, warning against steps that raise any doubts about US presidential authority in a conflict.

"One of the things that voters think about" in US presidential elections, Rubio said, "is whether or not they want to trust him with this capability."

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