Speaker Nancy Pelosi of California on Friday took the unprecedented step of asking the chairman of the Joint Chiefs of Staff about “available precautions” to prevent President Trump from initiating military action abroad or using his sole authority to launch nuclear weapons in the last days of his term.
In a phone call to the chairman, Gen. Mark A. Milley, Ms. Pelosi appeared to be seeking to have the Pentagon leadership essentially remove Mr. Trump from his authorities as the commander in chief. That could be accomplished by ignoring the president’s orders or slowing them by questioning whether they were issued legally.
But General Milley appears to have made no commitments. Short of the cabinet invoking the 25th Amendment or removing Mr. Trump through impeachment in the House and conviction in the Senate, it is unconstitutional to defy legal orders from the commander in chief.
Ms. Pelosi’s request, which she announced to the Democratic caucus as an effort to prevent “an unhinged president” from using the nuclear codes, was wrapped in the politics of seeking a second impeachment of Mr. Trump.
Col. Dave Butler, a spokesman for General Milley, confirmed that the phone call with the speaker had taken place but described it as informational. “He answered her questions regarding the process of nuclear command authority,” he said.
But some Defense Department officials clearly resented being asked to act outside of the legal authority of the 25th Amendment and saw it as more evidence of a broken political system. They said that some political leaders were trying to get the Pentagon to do the work of Congress and cabinet secretaries, who have legal options to remove a president.
Mr. Trump, they noted, is still the commander in chief; unless he is removed, the military is bound to follow his lawful orders. While military officials can refuse to carry out orders they view as illegal — or slow the process by sending those orders for careful legal review — they cannot remove the president from the chain of command. That would amount to a military coup, the officials said.
But two former administration officials with close ties to the national security establishment said that they had seen signs that Mr. Trump’s aides were, in the words of one, “routing around” the president by not raising issues that could prompt him to move toward military action.
The one issue that has worried officials the most is Iran’s announcement that it has begun enriching uranium to 20 percent purity — near the quality to make a bomb. In December, Mr. Trump asked for military options that might be taken in response to Iran’s escalating production of nuclear fuel, but he was talked out of it by a number of top officials, including General Milley and Secretary of State Mike Pompeo.
In a note to her Democratic majority, Ms. Pelosi said she had asked General Milley about “available precautions for preventing an unstable president from initiating military hostilities or accessing the launch codes and ordering a nuclear strike. The situation of this unhinged president could not be more dangerous, and we must do everything that we can to protect the American people from his unbalanced assault on our country and our democracy.”
Other Democrats took up the theme. “The president should be removed,” Senator Patrick J. Leahy of Vermont said. “I hate to think of him being down there having access to nuclear weapons and all that.”
This was not the first time the issue has come up in American history, or in regard to Mr. Trump.
In the last days of Richard M. Nixon’s presidency, the defense secretary, James R. Schlesinger, quietly issued a set of orders that if Mr. Nixon sought to move or use nuclear weapons, commanders should route the request to him or Secretary of State Henry A. Kissinger. Mr. Schlesinger, describing his actions only after Mr. Nixon left office, said he was concerned that the president was drinking, or that he might lash out.
Michael Beschloss, the presidential historian, said Mr. Schlesinger had told him a number of years ago that “he was worried about Mr. Nixon’s physical and emotional state and wanted to make sure there was no danger the nuclear arsenal would be abused.”
Mr. Schlesinger died in 2014; Mr. Kissinger, 97, said several years ago that he had not been aware of any such orders.
In the 2016 presidential campaign, Hillary Clinton also raised the issue of Mr. Trump’s suitability to command the nuclear arsenal. “Imagine him in the Oval Office facing a real crisis,” she said in her address at the Democratic National Convention. “A man you can bait with a tweet is not a man we can trust with nuclear weapons.”
During his presidency, Mr. Trump hinted at using nuclear weapons only once: when he was in his first standoff with Kim Jong-un of North Korea in 2017. He threatened “fire and fury like the world has never seen” and later told aides that he thought the threat forced Mr. Kim into diplomacy — though the series of three summit meetings with the North Korean leader led to none of the disarmament that he had predicted it would.
Now, at the end of Mr. Trump’s presidency, Ms. Pelosi is again raising the specter of an unstable leader as part of her effort to pressure Republicans to join in a second impeachment resolution — even if there is no time for a trial in the Senate.
Peter D. Feaver, a professor of political science at Duke University who has studied the armed forces, said the military could, in theory, physically restrict Mr. Trump’s access to the nuclear codes because it provides the command, control and communications that link the president to the nuclear arsenal. A military aide with the so-called nuclear football containing the launch codes is just feet from the president at all times.
But legally, the military cannot deny the president access to the codes unless the 25th Amendment has been activated.
“As long as President Trump is commander in chief, then one of the highest-priority missions the military has is maintaining connectivity between him and the nuclear arsenal, and I expect that is what they are doing and will continue to do until Inauguration Day,” Mr. Feaver said.
“This is a good example of people asking the military to solve a problem that is not the military’s to solve,” he added. “If Congress believes that President Trump should not have access to the nuclear codes, then Congress has the capacity to make that happen through impeachment.”
Mr. Feaver and other military specialists said on Friday that Mr. Trump could not carry out orders to fire nuclear weapons on his own because of a series of checks that are in place. For instance, the Pentagon can insist that orders come through the legal process, in writing, before they execute them.
“Under the hypothetical that Speaker Pelosi appears to be imagining, insisting that the order come through regular channels — and not the president calling on his iPhone — would feel like resisting an order, but it would actually be insisting that it be legal,” Mr. Feaver said.
He said the Pentagon could ask for a legal review from its own lawyers, the attorney general and others. “All of that would functionally accomplish what she is asking for and would be legal,” Mr. Feaver said.
David E. Sanger reported from Vermont, and Eric Schmitt from Washington. Helene Cooper contributed reporting from Washington.