"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 USA national security interests", Democrat Chris Murphy told the Senate Foreign Relations Committee.
The topic on the United States president's authority to launch a nuclear missile has not been discussed in almost four decades since a four-day hearing.
"One of the things that voters think about" in United States presidential elections, Rubio said, "is whether or not they want to trust him with this capability".
President Trump, who wrapped up a 13-day trip to Asia, previously called Pyongyang's nuclear ambitions a global threat and once warned Kim Jong Un that if North Korea threatened the U.S., America has "military solutions" "locked and loaded".
A U.S. B-52 bomber flies over Osan Air Base in South Korea.
"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 USA national security interests".
During a Senate hearing reviewing the powers of the president to order a nuclear strike, Senator Chris Murphy (D-CT) made clear what many Americans are thinking.
"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.
Republican Senators said second approval may get in the way of taking down a nuclear missile sent from an enemy state, noting that in the Cold War it could take only 30 minutes for a missile from Russian Federation to reach the U.S.
That explanation did not satisfy committee Democrats, who insisted Trump's behavior, and what they identify as his habit of nominating and hiring administration officials who defer to his worldview, means any internal resistance "does not offer real resistance if the president absolutely insists upon his way", Sen.
Sen. Edward Markey, D-Mass., still likened the process to simple technology.
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 three other Democrats have co-sponsored it.
No Trump administration officials are testifying before the hearing, which is examining the nuclear command and control structure that has served all USA presidents. "If anyone out there thinks they can somehow get away with something because the politics of the United States would somehow prevent the commander-in-chief from acting expeditiously, that could also encourage miscalculation, particularly on behalf of people who are isolated from the world, don't get a lot of information, and have never had anyone tell them they're wrong or no, and I have one person in particular in North Korea who concerns me in that regard".
Congress should step carefully when debating legislative controls to the president's ability to use nuclear weapons, the panelists cautioned. James E. Risch (R-Idaho) said.
Modern nuclear threats, like North Korea, tend to be more volatile but with less destructive capabilities.
Trump has used both confrontational and conciliatory language on North Korea in recent days.