The Massachusetts Democrat continued to press the issue and urged support for legislation, which would bar any American president from launching a nuclear weapon without congressional approval, as the Senate Foreign Relations committee heard testimony on nuclear weapons authority.
But Democrats on the committee were happy to invoke Mr Trump, and they noted that the President's escalating rhetorical battle with North Korea - a nuclear-armed nation he and his advisers have repeatedly threatened to annihilate - lent urgency to their questions about how, if at all, presidents are limited in their abilities to fire nuclear missiles. Indeed, a military aide shadows the commander in chief day and night, carrying the black briefcase commonly referred to as the "nuclear football", packed with attack options and other information needed in a national emergency.
While the President retains constitutional authority to order some military action, Kehler explained that the nuclear decision process "includes assessment, review and consultation between the President and key civilian and military leaders, followed by transmission and implementation of any Presidential decision by the forces themselves".
"No one human being should have the power to unilaterally unleash the most destructive forces ever devised by humankind".
The issue is particularly concerning to Congress because of heightened tensions with North Korea and the war of words between the nations' leaders.
Kehler responded "Yes, if there is an illegal order presented to military, the military is obligated to refuse to follow it".
During the hearing, members of the Senate Foreign Affairs Committee questioned longstanding presidential authority to deploy nuclear weapons.
"If we are under attack, the president would have the authority under Article 2 to defend the country and there's no distinction between his authority to use conventional or nuclear weapons in response to such an attack", McKeon told lawmakers.
President Donald Trump and his unpredictable, erratic approach to major crises has prompted the hearing.
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. Chris Murphy, Connecticut Democrat.
"We are concerned that the president 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", Mr Murphy said in Congress. "So let's just recognize the exceptional nature of this moment and the discussion that we're having today". He said would immediately have questioned such an order, but publicly questioned the inherent risks. Once given, the order can't be revoked, Mr. Corker said.
Brian McKeon, the former acting under-secretary for policy at the U.S. Department of Defense who testified before the panel, told Markey that he believes the Constitution would require a president to seek congressional approval to launch a nuclear strike in the absence of an attack or imminent threat. Even then, the president is not likely to "make this decision by himself". "They'd be asking questions that would slow down that process".