"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 weapon strike that is wildly out of step with U.S. national security interests", he added.
Senate Foreign Relations Committee Chairman Bob Corker said the examination was about the structure that allows presidents to make critical decisions.
"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.
Kehler acknowledged, however, should the military reject a nuclear strike order that would result in a "very hard conversation".
"One of the things that voters think about" in U.S. presidential elections, Rubio said, "is whether or not they want to trust him with this capability". "I don't think the assurances I've received today will be satisfying to the American people".
On Tuesday, former officials cautioned that adding Congress to the equation would hamper the U.S. response in a high-stress scenario without a lot of time. In a September speech at the United Nations, Trump said the United States would "totally destroy North Korea" if it had to.
Here's a question rarely raised before Donald Trump ran for the White House: If the president ordered a pre-emptive nuclear strike, could anyone stop him?
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The president, as commander in chief, is the sole arbiter or whether to use the US nuclear arsenal - an issue that hasn't been debated at the congressional level in more than 40 years.
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". Kehler, who led the agency responsible for nuclear launches, insisted on several occasions the military could refuse to act on any nuclear launch order it determined to be illegal - and there is time to push back against a president in any situation, apart from responding to an imminent attack. 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".
It was the first time since 1976 that Congress held a hearing on this topic. In this scenario, Mattis, national security adviser Lt. Gen. H.R. McMaster and Secretary of State Rex Tillerson are not part of the chain of command.
On Monday, Mattis was directly asked about the hearing and its implications. Asked whether he was comfortable with the system in its current form, he curtly answered "Yes, I am".
"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.