That puts him outside the orthodoxy embraced by many of his fellow Republicans but inside a widening circle of national security thinkers — including President Barack Obama — who believe nuclear weapons are becoming more a liability than an asset, less relevant to 21st century security threats like terrorism.

“Sen. Hagel certainly would bring to office a more ambitious view on nuclear reductions than his predecessors,” said Steven Pifer, a senior fellow at the Brookings Institution. “While he would likely take a less dramatic position in office, it might not be a bad thing to have a secretary of defense question what nuclear deterrence requires today.”

The customary stance of defense secretaries in the nuclear age has been that the weapons are a necessary evil, a required ingredient in American defense strategy that can be discarded only at the nation’s peril.

Hagel, 66, takes a subtly different view — one shared by Obama but opposed by those in Congress who believe disarmament is weakness and that an outsized American nuclear arsenal must be maintained indefinitely as a counterweight to the nuclear ambitions of anti-Western countries like North Korea and Iran.

Hagel argues for doing away with nuclear weapons entirely, but not immediately and not unilaterally.

In a letter to Obama two months after his former Senate colleague entered the White House in 2009, Hagelwrote that Global Zero was developing a step-by-step plan for achieving “the total elimination of all nuclear weapons,” but with a “clear, realistic and pragmatic appreciation” for the difficulty of realizing that goal.

Dozens of prominent politicians, diplomats and retired military leaders signed the letter. One month later Obama spoke in Prague of “a world without nuclear weapons,” while saying it might not happen in his lifetime. Obama declared that “as the only nuclear power to have used a nuclear weapon, the United States has a moral responsibility to act. We cannot succeed in this endeavor alone, but we can lead it, we can start it.”

Hagel, a Republican from Nebraska whose nomination has drawn heated criticism for his past statements on Israel, Iran and gays, is likely to also face questions on nuclear issues at his Senate confirmation hearing scheduled for Thursday. A Vietnam war veteran, he served in the Senate from 1997 to 2009.

The questions actually began last week at the confirmation hearing for John Kerry, Obama’s nominee for secretary of state. Sen. Bob Corker, R-Tenn., said he found Hagel’s affiliation with Global Zero “very concerning,” and he worried that Hagel’s views appeared to make him “very different than previous defense leaders.”

Kerry said he believes Hagel is a realist on the topic of nuclear arms reductions. But he also acknowledged that when he first heard about Global Zero’s central vision — the elimination of all nuclear weapons — “I sort of scratched my head and I said, ‘What? You know, how’s that going to work?’” But then he came to see this as nothing more than a long-range goal — “it’s not something that could happen in today’s world.”

Hagel, indeed, is thinking long term.

“Getting to global zero will take years,” Hagel wrote in the March 2009 letter to Obama on behalf of Global Zero. “So it is important that we set our course toward a world without nuclear weapons now to ensure that our children do not live under the nuclear shadow of the last century.”

Hagel stands out in this regard in part because history — first the demise of the Soviet Union, then the rise of terrorism as a global threat — has changed how many people think about the deterrent value of nuclear weapons. For decades after the birth of the atomic age in the 1940s the chief concern was controlling the growth, and later managing the shrinkage, of nuclear arsenals without upsetting the balance of power.

Today the thinking by many national security experts has shifted as the threat of all-out nuclear war has faded and terrorist organizations with potentially global reach, like al-Qaida, are trying to get their hands on a nuclear device.

“Hagel’s views reflect the growing bipartisan consensus in the U.S. security establishment that whatever benefits nuclear weapons may have had during the Cold War are now outweighed by the threat they present,” said Joe Cirincione, president of the Ploughshares Fund, which supports efforts to eliminate nuclear weapons.

Hagel was co-author of a Global Zero report last May that proposed, as an interim step, reducing the U.S. arsenal to 900 weapons within a decade, with half deployed and the other half in reserve. That compares with a current U.S. stockpile of 5,000, of which 1,700 are deployed and capable of striking targets around the globe.

The report said these cuts could be taken unilaterally if not negotiated with the Russians or carried out through reciprocal U.S. and Russian presidential directives. It called the unilateral approach “less good” but feasible. At a later stage China and other nuclear weapons countries would be brought to the table for negotiations on further cuts on the path to global zero, it said.

The White House last year weighed options for substantial new cuts in the number of deployed weapons, possibly to about 1,000 or 1,100 and probably as part of a negotiation with Moscow. But a decision, following a lengthy review of U.S. nuclear targeting requirements, was put off prior to the November election. Officials and private experts close to the administration believe Obama will soon embrace those cuts.

Previous secretaries of defense have supported reducing the U.S. nuclear stockpile under certain circumstances and have paid lip service to the United States’ commitment under the 1970 nuclear Non-proliferation Treaty to eventually eliminate its nuclear arms. But none has pushed these ideas like Hagel has.

“It’s historic,” said Bruce Blair, a co-founder of Global Zero and a former Air Force nuclear missile launch control officer.

“We will have, if he’s confirmed, a secretary of defense who’s committed to the sharp reduction of nuclear weapons, leading down a path toward their elimination,” Blair said in an interview last week. “I don’t think any sitting secretary of defense has ever come anywhere close to Hagel’s advocacy for this cause.”

Leon Panetta, the current defense secretary, has not taken a public stance on future nuclear reductions.

Some Pentagon chiefs, like William Perry, became public advocates for eliminating nuclear weapons after leaving office.

At least one apparently harbored doubts about the conventional wisdom while still serving.

In his 1995 memoir, Robert McNamara, who served as President John F. Kennedy’s defense secretary, wrote that by the time he entered the Pentagon in 1961 he had privately concluded that nuclear arms served no useful purpose. But he could not say that publicly, he wrote, because it contradicted established U.S. policy.