During Monday’s hearing, Gen. C. Robert Kehler, former commander of Strategic Command, emphasized that a presidential order to use a nuclear weapon must be legal. The basic legal principles of proportionality and necessity apply to the use of nuclear weapons, he said, and "if the order was considered to be illegal, the military is obligated to refuse to follow it."
“The U.S. military doesn’t blindly follow orders,” Kehler said in his opening remarks.
Democratic Senators were, nonetheless, concerned about the president’s previous remarks.
Witnesses at the hearing stressed that there are some checks. The system requires the president to work with military aides and give orders that must be followed down a chain of command.
Peter Feaver, professor of political science and public policy at Duke University, reminded the committee that “the system is not a button the president can accidentally lean on, against, on the desk and immediately cause nuclear missiles to fly.”
On Monday, Mattis was directly asked about the hearing and its implications.
“I’m the president’s principal adviser on the use of force,” he told reporters, confirming he is comfortable with the way the system works.
The committee’s Democrats also expressed concerns about whether the military could refuse to obey an order to launch nuclear weapons.
Cardin doubled down on his questioning of Kehler asking, “If you believe that this did not meet the legal test of proportionality, even if ordered by the president of the United States to use a nuclear first strike, you believe that because of legalities you retain that decision to disobey?”
Kehler responded “Yes, if there is an illegal order presented to military, the military is obligated to refuse to follow it.”
“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.”
ABC News’s Ali Rogin and Luis Martinez contributed to this report.