Is there any way to prevent President Donald Trump from impulsively starting a nuclear war after, say, some particularly heated late-night tweets? A dozen U.S. senators would like to know.

Trump-critic Bob Corker, a Tennessee Republican, convened a hearing of the Foreign Relations Committee on Tuesday morning to discuss America's nuclear chain of command. "The decision to use nuclear weapons is the most consequential of all," Cooker said. "I would like to explore the realities of this system."

His Democratic colleagues were less circumspect. "We are concerned that the president of the United States is so unstable, so quixotic in his decisionmaking that he could start a nuclear war," said Sen. Scott Murphy, a Connecticut Democrat. "Let's recognize the exceptional nature of this moment."

Under current law, the president alone can order a nuclear assault — and there are no clear checks on that authority. Among atomic powers, only the United States, France and North Korea give their top leaders sole nuclear-launch authority. A NATO country recently “raised concerns” about Trump’s nuclear authority, CNN reported.

Article II of the U.S. Constitution gives the president power to defend the United States from attack. Since the beginning of the Cold War, scholars and lawmakers have interpreted the article as giving the president the authority to launch a retaliatory nuclear barrage on short notice and without consulting Congress.

But experts at Corker's hearing said it's less clear that the president can, all on his own, legally order the military to nuke a country that hasn't already launched its own atomic weapon at the United States. Trump has threatened to "totally destroy" North Korea with "fire and fury," a threat that could imply unilateral nuclear action.

But "Article II does not give him carte blanche to take the country to war," Brian McKeon, a former Defense Department undersecretary, said at the hearing.

The 1973 War Powers Act requires Congressional approval for major military action, but presidents have routinely sidestepped that law. C. Robert Kehler, a retired Air Force general and former head of U.S. nuclear forces, told Corker's committee there's another, arguably more important check on a president's power.

"The U.S. military does not blindly follow orders," Kehler said, adding that any command to launch nuclear weapons must be proportional to the threat in order to be legal.

But there are no simple guidelines for military commanders rejecting an illegal order. Kehler said that as America's top nuclear command, he would have refused to launch an unprovoked nuclear attack. But he admitted that there's no clear legal framework for saying no to a trigger-happy president. "That would be a very difficult conversation."

For generations, the political and military establishments have relied on custom, not law, to guide a president's nuclear decision-making. In an atomic crisis, "what would matter most is the human element," Peter Feaver, a political science professor at Duke University, said at the hearing.

Feaver said a president's advisors — potentially including the vice president, cabinet and senior military leaders — should be able to talk the commander-in-chief out of an unnecessary nuclear attack.

But Trump is known to make major decisions without the knowledge of many of his advisors. Yesterday's norms might not constrain today's president — and Sen. Ed Markey, a Massachusetts Democrat, said that terrifies him and his constituents. "Many Americans share my fear that the president's bombastic words could turn into nuclear reality."

There are no easy solutions to the nuclear-command dilemma. Markey has proposed legislation reasserting Congress's power to veto any unprovoked nuclear attack. But the bill has little chance of attracting significant numbers of Republican votes.

A few Republican members of Corker's committee insisted even talking about nuclear-command reform could project an image of American weakness. "Our allies are watching and we don't want to give them any reason to doubt our commitment" to mutual defense, Florida senator Marco Rubio said at the hearing.

"Our adversaries are watching,” Rubio added. Corker’s hearing “could encourage miscalculation.”

"We need to be careful in how we discuss this," Corker agreed.

Markey dismissed objections, saying lawmakers shouldn’t be shy to speak up about  the commander in chief’s power to end the world.

"No one can tell the president no," he said. "It boggles the rational mind."