House Speaker Nancy Pelosi said Friday she spoke to Joint Chiefs of Staff Chairman Gen. Mark Milley about precautions that could block "an unstable president" from "ordering a nuclear strike," or even accessing nuclear launch codes or starting other military hostilities.
"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," Pelosi said in a letter to fellow Democratic lawmakers.
The California Democrat later told lawmakers at a meeting that Milley had assured her that there are currently steps that would prevent President Donald Trump from ordering a nuclear launch.
A spokesman for the Office of the Joint Chiefs said Pelosi initiated a call with Milley and "he answered her questions regarding the process of nuclear command authority."
The Pentagon did not respond to CNBC's requests for comment. The National Security Council declined to comment.
Pelosi's letter comes two days after a mob of Trump supporters rioted at the U.S. Capitol after the president encouraged them to "fight" to prevent Joe Biden from becoming the president. Five people, including a Capitol police officer, died in the riot.
The section of the letter referring to her conversation with Milley is entitled, "Preventing an Unhinged President From Using the Nuclear Codes."
"This morning, I spoke to the Chairman of the Joint Chiefs of Staff Mark Milley to discuss available precautions for preventing an unstable president from initiating military hostilities or accessing the launch codes and ordering a nuclear strike," Pelosi wrote.
The joint chiefs' vice-chairman, Air Force Gen. John Hyten, said during a speech three years ago that he would resist executing an order from Trump to launch nuclear weapons if he considered such an order "illegal."
Pelosi, Senate Minority Leader Chuck Schumer of New York and other Democratic lawmakers have demanded that Trump be removed from office, either through invocation of the Constitution's 25th Amendment or via impeachment.
In her letter, Pelosi told lawmakers that, "Nearly fifty years ago, after years of enabling their rogue President, Republicans in Congress finally told President [Richard] Nixon that it was time to go."
"Today, following the President's dangerous and seditious acts, Republicans in Congress need to follow that example and call on Trump to depart his office – immediately," she added. "If the President does not leave office imminently and willingly, the Congress will proceed with our action."
James Schlesinger, who was Nixon's secretary of Defense, has said that during the final days of the Watergate crisis, he ordered military commanders to contact him or Secretary of State Henry Kissinger if Nixon directed a nuclear launch.
At the time, Nixon was drinking heavily as he lost public and congressional support.
During one meeting, Nixon told a group of congressmen, "I can go into my office and pick up a telephone and in 25 minutes millions of people will be dead," according to the book "The Final Days" by reporters Bob Woodward, and Carl Bernstein.
That led Sen. Alan Cranston, D-Calif., to warn Schlesinger about "the need for keeping a berserk president from plunging us into a holocaust."
Trump's former Defense secretary, James Mattis, has blamed the president for Wednesday's riot, saying Trump had used his office to "destroy trust in our election and to poison our respect for fellow citizens."
Earlier in the week, Mattis and the nation's nine other living former Defense secretaries, warned in a Washington Post op-ed that the U.S. military should have no role in determining election outcomes.
"Each of us swore an oath to support and defend the Constitution against all enemies, foreign and domestic. We did not swear it to an individual or a party," they wrote.
The group also called on Trump's acting Defense secretary, Christopher Miller, and other political appointees and civil servants to "refrain from any political actions that undermine the results of the election or hinder the success of the new team."