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Allies of Donald J. Trump are proposing that the United States restart the testing of nuclear weapons in underground detonations should the former president be re-elected in November. A number of nuclear experts reject such a resumption as unnecessary and say it would threaten to end a testing moratorium that the world’s major atomic powers have honored for decades.

In the latest issue of Foreign Affairs magazine, Robert C. O’Brien, a former national security adviser to Mr. Trump, urges him to conduct nuclear tests if he wins a new term. Washington, he wrote, “must test new nuclear weapons for reliability and safety in the real world for the first time since 1992.” Doing so, he added, would help the United States “maintain technical and numerical superiority to the combined Chinese and Russian nuclear stockpiles.”

At the Cold War’s end, in 1992, the United States gave up the explosive testing of nuclear arms and eventually talked other atomic powers into doing likewise. The United States instead turned to experts and machines at the nation’s weapons labs to verify the lethality of the country’s arsenal. Today the machines include room-size supercomputers, the world’s most powerful X-ray machine and a system of lasers the size of a sports stadium.

In his article, Mr. O’Brien described such work as just “using computer models.” Republican members of Congress and some nuclear experts have faulted the nonexplosive testing as insufficient to assure the U.S. military establishment that its arsenal works, and have called for live tests.

But the Biden administration and other Democrats warn that a U.S. test could lead to a chain reaction of testing by other countries. Over time, they add, resumption could result in a nuclear arms race that destabilizes the global balance of terror and heightens the risk of war.

“It’s a terrible idea,” said Ernest J. Moniz, who oversaw the U.S. nuclear arsenal as the secretary of energy in the Obama administration. “New testing would make us less secure. You can’t divorce it from the global repercussions.”

Siegfried S. Hecker, a former director of the Los Alamos weapons lab in New Mexico where J. Robert Oppenheimer led the creation of the atomic bomb, called new testing a risky trade-off between domestic gains and global losses. “We stand to lose more” than America’s nuclear rivals would, he said.

It’s unclear if Mr. Trump would act on the testing proposals. In a statement, Chris LaCivita and Susie Wiles, Mr. Trump’s co-campaign managers, did not directly address the candidate’s position on nuclear testing. They said that Mr. O’Brien as well as other outside groups and individuals were “misguided, speaking prematurely, and may well be entirely wrong” about a second Trump administration’s plans.

Even so, Mr. Trump’s history of atomic bluster, threats and hard-line policies suggests that he may be open to such guidance from his security advisers. In 2018, he boasted that his “Nuclear Button” was “much bigger & more powerful” than the force controller of Kim Jong Un, the North Korean leader.

A U.S. detonation would violate the Comprehensive Test Ban Treaty, long considered one of the most successful arms control measures. Signed by the world’s atomic powers in 1996, it sought to curb a costly arms race that had spun out of control.

During the Cold War, China set off 45 test explosions, France 210, Russia 715 and the United States 1,030, with the goal of uncovering flaws in weapon designs and verifying their reliability.

Nuclear experts say that the test disparities give Washington a military edge because it keeps other powers from making their arsenals more diverse and deadly.

In 2017, Mr. Trump’s presidential inauguration revived the possibility of new testing. In addition to discussing a restart, officials in his administration called for reductions in the preparation time for a U.S. nuclear test resumption. The federal agency in charge of the nation’s nuclear test site ordered the required time for preparations to drop from years to as little as six months.

Nuclear experts saw the goal as unrealistic because testing equipment at the sprawling site, in the Nevada desert, had fallen into disrepair, or vanished.

Last year, the Heritage Foundation, a conservative think tank, recommended that the United States eliminate the preparation time. Its policy guide for conservative presidential candidates called on Washington “to move to immediate test readiness.”

In his Foreign Affairs article, Mr. O’Brien argued that the Biden administration had responded weakly to Chinese and Russian buildups of nuclear arms. The explosive testing of American arms, he said, would strengthen the U.S. arsenal and help deter America’s foes. His article zeroed in on the safety and reliability of new designs, not ones tested during the Cold War.

“It would be negligent to field nuclear weapons of novel designs that we have never tested in the real world,” said Christian Whiton, who served as a State Department adviser in the George W. Bush and Trump administrations and provided background research for Mr. O’Brien’s article.

Asked for examples, Mr. Whiton cited two new American weapons that he said were in need of explosive testing. Both are thermonuclear weapons, also known as hydrogen bombs. And both have a destructive force that is many times more powerful than the bomb that leveled Hiroshima.

The first of the cited bombs, the W93, is to fit atop submarine missiles. The Biden administration announced its development in March 2022, and Mr. Whiton called it “a completely new design.”

But the Biden administration’s work plan for the W93 says otherwise. The warhead, it notes, will rely “on currently deployed and previously tested nuclear designs.” Moreover, its makers, at the Los Alamos lab, have insisted that the warhead can be fielded safely and reliably without recourse to more explosive tests.

Charles W. Nakhleh, the lab’s associate director for weapons physics, said in a Los Alamos publication that the alternatives to live detonations “will enable us to field the W93 without needing any additional nuclear testing.”

The other weapon Mr. Whiton cited is the B61-13, a variation of a bomb first deployed in 1968. The Biden administration announced its development in October, and Mr. Whiton called it “heavily redesigned.” Even so, the official plan says that its nuclear parts are to be salvaged from an older B61 version and recycled in the new model.

“The idea that it’s a big redesign doesn’t hold water,” said Hans M. Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, a private research organization in Washington. “They’ve already tested the part that goes bang.”

Mr. Whiton, however, believes that even modest changes “ought to be proved in the real world.” He also argued that the United States would have to develop new warheads to counter an emerging class of superfast weapons — known as hypersonics — that China and Russia are developing. “It’s likely that new warhead designs will be necessary,” he said, and will require new testing.

Despite the conflicting claims and uncertain election outcomes, nuclear experts say that China and Russia are readying their test sites for new detonations, perhaps in case the United States restarts its program, or alternatively to race ahead on their own. Dr. Moniz, the former secretary of energy, said he fears that Washington will go first if Mr. Trump wins a second term.

Mr. Whiton, the former State Department adviser, cast doubt on the idea that a U.S. detonation would set off a global chain reaction. He noted that Russia and China were already building up their arsenals without recourse to new testing.

“It is unclear if existing and aspiring nuclear states would follow us,” he said of a global reaction. “If they do, the downside is that they might improve their capabilities marginally.”

The upside, Mr. Whiton said, is that the United States could study the foreign detonations for clues about their hidden characteristics. It could, for example, monitor the faint rumbles in bedrock from an underground test to estimate a device’s power.

Mr. Whiton added that such readings would, in turn, “help us update our deterrent properly.”

The trouble with Mr. Whiton’s point, a number of nuclear experts say, is its unstated corollary: that the world could descend into the rounds of costly moves and countermoves that characterized the Cold War. In this century, they warn, a nuclear arms race could prove to be more global, innovative, deadly and unpredictable.

“China has much more to gain from resumed testing than we do,” said Dr. Hecker, the former Los Alamos director. “It would open the door for others to test and reignite an arms race to the peril of the entire world. We shouldn’t go there.”

Michael Gold contributed reporting.