Under current rules, a president could launch a nuclear strike by entering codes into a device known as "the football" - a briefcase that always travels with the president - and is not obligated to consult other government officials.
"Then what happens?" asked Sen.
Marco Rubio, who ran against Trump in the Republican presidential primary a year ago, was among lawmakers who were quick to point out that the hearing should not be taken as a reduced USA nuclear posture. "It boggles the rational mind", said Markey, who has sponsored legislation that would prevent any president from launching a first strike.
He quickly followed that up by saying "I fear that in the age of Trump, the cooler heads and strategic doctrine that we once relied upon as our last best hope against the unthinkable seem less reassuring than ever".
Experts say Trump has unchecked authority to launch a conventional or nuclear attack on North Korea. "Even General Kelly, the president's chief of staff, can't control the president's Twitter tantrums".
Tuesday's hearing reflected the "exceptional nature" of the present context, said Senator Chris Murphy, a Democrat from CT.
Other former national security officials testified that if there isn't an imminent attack, it would be more hard for the president to launch a nuclear attack out of the blue.
"The president would not make this decision by himself", said Brian McKeon, a former acting undersecretary for policy with the Department of Defense.
"It wouldn't be the president alone persuading a single military officer alone on the other side of the telephone", he said.
A North Atlantic Treaty Organisation partner country raised concerns about the President's command of the United States launch system. A North Korean official dubbed a tweet by Mr Trump a "declaration of war" after the president said that North Korean officials "won't be around much longer" if they continue with their escalating rhetoric over Pyongyang's nuclear program.
HRC: New FBI hate-crimes statistics "harrowing"
It was the second year in a row the number of reported hate crimes has increased, after a slight decrease from 2013 to 2014. The community has been a victim of hate crimes as they are usually mistaken as Arabs due to their turbans.
On Tuesday, former officials cautioned that adding Congress to the equation would hamper the United States response in a high-stress scenario without a lot of time.
But one expert told the committee Tuesday such an order could, under certain circumstances, be blocked.
Adding the extra layer would lead to "conflicting signals" that "can result in loss of confidence, confusion or paralysis in the operating forces at a critical moment", Kehler said.
"This continues a series of hearings to examine these issues and will be the first time since 1976 that this committee or our House counterparts have looked specifically at the authority and process for using USA nuclear weapons", Corker, who is from Tennessee, said in his statement. Gerald Ford was president.
"Under existing laws, the president of the United States can start a nuclear war - without provocation, without consultation, and without warning". The deterrent strength of a nuclear arsenal is not only in its ability to strike anywhere, but in the ability of the Commander-in-Chief to expeditiously make that decision.
"The system for decision is created to ensure the president consults with the national security council and his other senior civilian and military advisers and I would expect that to occur in every case where the use of nuclear weapons is contemplated".
"Many interpret that to mean that the president is actively considering the use of nuclear weapons in order to deal with the threat of North Korea".
Pressed by reporters on Monday, Mattis refused to comment on whether he would be excluded from the chain of command when it comes to launching a nuclear strike. "I think it will be very informative to the American people and the rest of the Senate about what powers the President has - should, shouldn't have, whatever".
"I don't have confidence that a military chain of command would reject an order by the president to launch nuclear weapons in a preventative nuclear war situation", Markey told the Guardian after Tuesday's hearing.
"I think that we have to keep trust, keep faith in the system that we have that has proven effective now for decades", he said.