Critics in the Senate have posed a high-stakes question: Can anything keep President Trump from launching a nuclear attack on his own?

"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 U.S. national security interests," said Connecticut Democrat Chris Murphy.

His Massachusetts colleague Ed Markey has offered legislation that would require congressional approval for any first use of nuclear weapons.

Markey told witnesses at a Senate Foreign Relations Committee hearing this week that he has been asking Republican Chairman Bob Corker of Tennessee for weeks to convene the session because of how important he considers this question.

And the answer — as to whether anything could stop Trump from ordering an attack — is yes, witnesses said.

The difficult part is what comes next.

Retired Air Force Gen. Robert Kehler, a former top commander of U.S. strategic forces, stressed to senators that under the military's current system, only the president can order a nuclear attack. But he acknowledged under questioning from senators that there might be some exceptions.

"Even if ordered by the president of the United States to use a nuclear first strike, you believe that under, because of legalities, you retain that decision to disobey the commander in chief?" asked Maryland Democrat Ben Cardin.

"Yes," Kehler said.

That prompted more questions from Wisconsin Republican Ron Johnson, who asked that if someone in Kehler's position felt that a president's order wasn't legal and decided not to follow his order to attack — "then what happens?"

Kehler: "Well, you know ... as I say — I don't know."

The result, all parties agreed, would be a constitutional crisis. And Republican allies of Trump not only dismiss the idea he is so unbalanced he can't be trusted with his finger on the button — they argue these kinds of allegations undercut America's position in the world.

Debating Trump's fitness to order a nuclear strike, Idaho Republican Jim Risch warned his colleagues, could send North Korea the wrong signals.

"Pyongyang needs to understand that they're dealing with a person who's commander in chief right now, who is very focused on defending this country and he will do what is necessary to defend this country," he said.

Other witnesses pointed out the need to clarify exactly what problem Congress might address. It makes sense for Trump to keep his ability to respond quickly to a nuclear attack on the United States, as compared with the issue of him, or another president, deciding independently to use strategic weapons first.

"Distinguish between the scenarios where the military wakes up the president versus scenarios where the president's waking up the military," said Peter Feaver, a political science professor from Duke University.

For example: "Where the president's waking up the military, maybe in an extreme funk, saying, 'I'm angry and I want something done.' In that setting,
the president alone could not effect the strike. He would require lots of people cooperating with him to make the strike happen, and they'd be asking the questions that would slow down that process."

Markey, whose legislation likely has little chance to pass in the Republican-controlled Congress, said he had not been comforted by the testimony at the hearing.

"I don't think that the assurances that I've received today will be satisfying to the American people," he said. "I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account."

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