Nuclear security experts are nervous about the prospect of a Donald Trump Donald John TrumpBiden leads Trump by 36 points nationally among Latinos: poll Trump dismisses climate change role in fires, says Newsom needs to manage forest better Jimmy Kimmel hits Trump for rallies while hosting Emmy Awards MORE presidency.

Former officials from both Republican and Democratic administrations are expressing concern over what they describe as Trump’s cavalier rhetoric about using nuclear weapons and potentially allowing them to be obtained by U.S. allies.

ADVERTISEMENT

“This is the most dangerous thing that he has said, among many dangerous and stupid things,” said Mark Fitzpatrick, 26-year veteran of the State Department who worked on nuclear nonproliferation issues under former Presidents Bill Clinton William (Bill) Jefferson ClintonBattle lines drawn on precedent in Supreme Court fight Sunday shows - Ruth Bader Ginsburg's death dominates Bill Clinton on GOP push to fill Ginsburg vacancy: Trump, McConnell 'first value is power' MORE and George W. Bush.

At the Pentagon on Thursday, President Obama responded directly to concerns that Trump was woefully unprepared to man the United States’ nuclear arsenal.

“Just listen to what Mr. Trump has to say and make your own judgment with respect to how confident you feel about his ability to manage things like our nuclear triad,” Obama said, referring to the three-pronged set of air, land and sea defenses that have been the backbone of nuclear weapons policy for a generation. During a Republican primary debate, Trump appeared not to be aware of the concept.

Earlier in the week, Obama called Trump “unfit” to serve as commander in chief, and his comments have been underlined by some Republicans.

Former Sen. Gordon Humphrey (R-N.H.) described Trump as a “sociopath” in an email to NBC News, calling him “pathologically insecure.”

“To imagine Trump in charge of our armed forces at a moment of crisis is frightening,” Humphrey added.

Trump allies say the characterizations made by Clinton and opponents of Trump are unfair.

Roger Stone, the longtime ally of Trump, said the new accusations are echoes of longtime Democratic efforts to portray Republican candidates as prone to using nuclear weapons.

He noted the famous “Daisy” ad aired by President Lyndon Johnson’s campaign in 1964 that cast Barry Goldwater, a Republican, as too dangerous to hold the keys to the nuclear arsenal.

“We have always used nuclear weapons as a deterrent, Trump is in that exact same tradition,” Stone said. “What are they trying to say? That he's trigger happy, he's mentally unstable and he would push the button? Nonsense.”

Stone also blasted Clinton's hawkish views on national defense, citing her record of support for the Iraq War to turn the accusations back around.

“He's the peace candidate. She's more likely to use the arsenal than he is. She supported war when he's been opposed to war,” Stone said. “He has a lovely family that he has no interest in destroying in a nuclear holocaust. It's an absurd notion.”

The Hill spoke to more than a half-dozen nuclear weapons experts for this story. All expressed a level of concern or anxiety about Trump’s control of nuclear weapons and his leadership of global nonproliferation.

“This could really trigger nuclear wars that could end mankind. Is that what he wants?” said Fitzpatrick, the executive director of the U.S. office of the International Institute for Strategic Studies, who has been publicly critical of Trump. “Talking about nuclear weapons the way he talked about it is not rational.”

Trump’s position on nuclear weapons were scrutinized again this week, following claims by MSNBC “Morning Joe” host Joe Scarborough, a former Republican member of Congress, that the GOP nominee had repeatedly asked a foreign policy briefer why the U.S. could not use its vast nuclear arsenal.

The Trump campaign has denied the report, and Trump aides characterized Scarborough's accusation as a political smear.

“Unless he can produce the name of said adviser, I'm calling bullshit on this,” Stone said.

In explaining their own worries about Trump, many experts who spoke to The Hill pointed to comments the candidate made suggesting that Japan, South Korea and maybe even Saudi Arabia should obtain nuclear stockpiles of their own.

“If Japan had that nuclear threat, I’m not sure that would be a bad thing for us,” Trump said in an interview with The New York Times in March.

The comments undermined a core tenet of U.S. nuclear policy for the last several decades: that fewer countries, not more, should have nuclear weapons.

The U.S. has been willing to provide its nuclear umbrella to countries such as Japan and South Korea, mainstream thinking goes, in order to discourage them from acquiring weapons of their own.

“If the United States were to reduce our defense commitment to Japan or South Korea and give either one of those countries, basically, a green light to pursue nuclear weapons, in my view it would vastly undermine the security situation in the region, increase the threat posed by nuclear weapons and, ultimately, to the United States,” said Kingston Reif, the director for disarmament and threat reduction policy at the Arms Control Association. Reif told The Hill he is not politically involved this election cycle.

If Japan were to acquire nuclear weapons, South Korea might not be far behind. And then both China and North Korea might feel compelled to increase their nuclear arsenals.

“Every president since Eisenhower has pursued a policy of trying to prevent the spread of nuclear weapons, and that’s Republican and Democrat,” said Gary Samore, Obama’s former White House coordinator for arms control and weapons of mass destruction and an adviser for Clinton’s campaign.

“It’s a very well-established article of faith among both parties that the U.S. should try to prevent additional countries from developing nuclear weapons.”

Trump appeared to directly rebut that argument in his Times interview. Not only was the prospect of greater proliferation inevitable, he suggested, but the current situation was overextending the U.S.’s resources.

“Biggest problem, to me, in the world, is nuclear, and proliferation,” Trump said. “At the same time, you know, we’re a country that doesn’t have money.”

The positions fit neatly within Trump’s “America First” ideology that has earned criticism from both sides of the aisle. The U.S.’s security commitments abroad have come at the expense of its domestic activities, he has claimed while questioning global security agreements and organizations.

Last month, Trump caused a stir by refusing to unconditionally pledge to come to the aide of NATO allies in Europe were they to be invaded by Russia.

Comments like that will force other countries to question the U.S.’s commitment to their security, said Douglas Feith, a high-ranking Pentagon official under President George W. Bush, potentially encouraging them to explore amassing nuclear weapons.

“Even though they don’t directly relate to nuclear weapons, they have an effect on nuclear weapons,” said Feith, who does not support Trump’s White House bid. “Because if our allies believe that he is unreliable, that he is not a faithful ally, then they won’t rely on our nuclear umbrella and they will possibly come to the conclusion that the only way they can have security is to develop a nuclear arsenal of their own.”

“When it comes to nuclear weapons, [lack of] message discipline is a policy weakness,” added James Acton, co-director of the nuclear policy program at the Carnegie Endowment for International Peace, who is unaligned in the presidential race.

Trump has been willing to talk more openly about the possibility of using nuclear weapons in a future conflict than other politicians before him.

Not only might he be willing to use nuclear weapons in a conflict in Europe, he has said, but he might employ them in a fight against the Islamic State in Iraq and Syria (ISIS).

“Somebody hits us within ISIS, you wouldn't fight back with a nuke?” he said on MSNBC in March.

The suggestion is a dramatic escalation in the conception of how nuclear weapons are used.

While the U.S. wields its thousands of nuclear missiles as a deterrence mechanism and reserves the right to use them as a first strike, it has long sought to avoid deploying the weapons except as an absolute last resort against a major threat to a large portion of the population or our allies. Being willing to launch a nuclear bomb against ISIS — which has never successfully directed an attack within the United States, much less proven capable of the kinds of wholesale destruction that might in other circumstances trigger a nuclear response — would be unprecedented.

Trump has repeatedly tried to maintain an aura of unpredictability, claiming that there is power in keeping adversaries guessing. But there’s also a risk, his critics claim, if no one knows when the line is crossed.

“Uncertainty in this business is a dangerous thing,” said John Noonan, a former national security adviser past presidential contenders Jeb Bush and Mitt Romney who has opposed Trump’s campaign, on MSNBC this week. Noonan has signed an open letter from dozens of Republican national security officials pledging not to support Trump.

“It's fundamentally dangerous.”