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.
"Let's just recognize the exceptional nature of this moment, the discussion that we're having today", he declared.
The hearing comes at a time of high tension between the United States and North Korea.
The question of whether anyone could stop a president from ordering a pre-emptive nuclear strike was rarely raised before Donald Trump ran for the White House.
Markey, who urged support for proposed limits, added that his concerns about a president's authority to launch a nuclear first strike are even more elevated given the Trump Administration's approach to foreign policy.
Stephen Schwartz, a nuclear weapons policy expert and the editor of "Atomic Audit", which assesses the costs of the USA nuclear weapons program, said in an interview that the nation is closer than it ever has been in the post-Cold War era to a miscalculation that could lead to nuclear war.
Senators trying to prevent President Donald Trump from launching an unprovoked nuclear attack were stymied Tuesday, after a panel of experts warned them against rewriting laws to restrain a commander in chief many worry is impulsive and unpredictable enough to start a devastating worldwide crisis.
Amid growing anxiety that President Donald Trump's heated rhetoric could trigger a war with North Korea, lawmakers on Tuesday debated for the first time in 40 years a U.S. president's authority to launch a nuclear strike.
When the military wakes the president, there is a significant possibility of an imminent strike and the president must quickly decide whether or not to deploy a nuclear counterstrike.
But one expert told the committee Tuesday such an order could, under certain circumstances, be blocked.
"It boggles the rational mind", said Sen.
The witnesses described teams of legal advisors working for the Commander of Strategic Command General John Hyten and in the office of the Secretary of Defense James Mattis who would advise the chain of command on the legality of a hypothetical order to use nuclear weapons.
"I don't think the assurances I've received today will be satisfying to the American people, I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account without a check and balance". Chris Murphy, D-Conn., one of several senators exploring how to prevent the president from launching a first nuclear strike without the permission of Congress.
But they acknowledged that the President could overrule the advice of his advisers and order a nuclear strike if it is deemed lawful.
But he is not legally obligated to consult with anyone.
The comments struck at the heart of the reason the hearing took place. Many in Congress believe there should be a mechanism for the president to get their approval before launching. Asked whether he was comfortable with the system in its current form, he curtly answered "Yes, I am". "But I would like to explore ... the realities of this system", he said.
"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.