The hearing was convened by Tennessee Republican Bob Corker, who has broken with the President in a public and acrimonious spat that has divided the former allies.
"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-weapons strike that is wildly out of step with U.S. national security interests", Democrat Chris Murphy told the Senate Foreign Relations Committee.
"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. He said those comments are fueled by Trump's statements about North Korea, including his remark in August that the US could respond to Pyongyang with "fire and fury like the world has never seen".
That was the message Tuesday from an extraordinary, first-time-in-four-decades hearing convened by Senate Foreign Relations Committee Chairman Bob Corker ― who last month said he anxious that Trump could "start World War III" and called the White House an "adult day care center".
Despite that dire warning, Mr Corker deliberately did not couch a hearing of the committee he chairs on presidential nuclear powers as a rebuke to Mr Trump, saying "this is not specific to anybody".
Kim, no stranger to verbal jousting, branded Trump a "dotard".
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.
But "because even a single nuclear detonation would be so consequential and might trigger an escalatory spiral that would lead to civilization-threatening outcomes, we must also have a high assurance that there would never be an accidental or unauthorized use of nuclear weapons", Feaver added.
Sen. Ben Cardin, a Maryland Democrat, said he's been receiving "more and more questions" during town hall meetings with constituents about whether the president can order a nuclear attack without any controls.
"If we saw they were preparing to do so and it was imminent, I could imagine it". In such a scenario, the timespan to deliberate is shorter and less subject to legal discussion, Feaver said.
Former Commander of US Strategic Command Gen C Robert Kehler noted that America's system requires civilian oversight, meaning "This is a system controlled by human beings".
Kehler acknowledged, however, should the military reject a nuclear strike order that would result in a "very hard conversation". Kehler, a retired US Air Force general, said the military principles of "necessity" and "proportionality" also apply to decisions about nuclear weapons.
"I don't know exactly", he responded.
"The president would not make this decision by himself", said Brian McKeon, a former acting undersecretary for policy with the Department of Defense. "But you'd have a real constitutional crisis on your hands if that occurred", he said.
"Our adversaries are watching", said Senator Marco Rubio, warning against steps that raise any doubts about United States presidential authority in a conflict. "I think that we have to keep trust, keep faith in the system that we have that has proven effective now for decades".