WASHINGTON — Ashton B. Carter, a Pentagon veteran and longtime Harvard University faculty member, emerged Tuesday as President Obama’s leading choice to replace outgoing Secretary of Defense Chuck Hagel as the White House seeks a trusted hand to steer defense policy in a period of deep uncertainty.

Carter, 60, held top Pentagon posts in the Clinton and Obama administrations in between a nearly two-decade career at Harvard’s Kennedy School of Government and MIT, where he was one of the leading thinkers on national security and helped train a generation of practitioners.

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The White House, which was said to be vetting the nomination, has not made a final decision, but Carter is the top contender, according to people on Capitol Hill familiar with the president’s thinking on the matter. In addition, The New York Times reported that unnamed administration officials identified Carter as the leading candidate for the job.

“If he’s nominated, it should be a pretty easy confirmation,” said Senator James Inhofe of Oklahoma, the top Republican on the Senate Armed Services Committee.

Carter has a unique pedigree that includes a doctorate in physics and a degree in medieval history, as well as service as the Pentagon’s second-ranking official under Hagel until last December. He was also the Pentagon’s top weapons buyer in Obama’s first term, giving him a unique mix of attributes at a time when defense budgets are shrinking but new threats are evolving from terrorist organizations like the Islamic State in Iraq and Syria. Carter is also an expert on US-Russian security relations.

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“This is not a time for learning on the job,” said Graham Allison, director of the Kennedy School’s Belfer Center for Science and International Affairs and a longtime Carter confidant. “Who else knows enough to do the job without spending a lot of time learning?”

But Carter, even though he has served all three of Obama’s defense secretaries — Hagel, Leon Panetta, and Robert Gates — is not considered a member of the president’s inner circle.

He rose to the top after several others with closer personal ties to the president said they were not interested in the Pentagon job.

That could prove challenging, according to Edward Luttwak, a longtime Pentagon consultant.

“That means interacting with other agencies under the direction of the White House,” he said. “And the White House, as everybody knows by now, is headed by a staff that has gotten the president into trouble time and time again.”

Carter, who has been lecturing at Stanford University this fall, seems aware of such pitfalls. When he returned to the Pentagon at the beginning of the Obama administration in 2009, he said that “Washington jobs can be miserable if you don’t have a good boss.”

Gary Samore, who served as a top White House security adviser in the first term, said he believes Carter and Obama will forge a good partnership.

“I have seen Ash and Obama together on a number of occasions,” Samore said. “The president seems to value his advice and intellect. But I think the biggest challenge Ash faces is prosecuting the war against the Islamic State.”

Carter grew up in a working-class family in Philadelphia and as a teenager pumped gas and repaired cars. He attended Yale University, where he quickly became an accomplished, wonkish scholar.

He studied medieval history and wrote his senior thesis on “the use of Latin by contemporary monastic writers to describe the vibrant world of 12th-century Flanders,” as Carter recalled in a brief faculty profile he wrote for Harvard. “I was an intensely serious student, what would probably be called today a ‘grind.’ ”

Carter was selected to be a Rhodes Scholar and studied at Oxford University, where he pursued a doctorate in theoretical physics. “My arrogant view at the time was that life would eventually teach me political science, sociology, psychology, and even economics, but it would never teach me linear algebra,” he later wrote.

In the decades since, he has pivoted between the theoretical jousting of the lecture hall to the work of a consummate Pentagon insider immersed in the details of high-tech weaponry.

In the Pentagon he was nicknamed “The Deliverer” for his ability, from behind a large oak desk that once belonged to legendary World War II General George S. Patton, to bust through red tape to get much-needed equipment to troops in the field.

In his last job, which he held for more than two years before stepping down last December, he was the day-to-day manager of the Department of Defense. From 2009 to 2011, in the post of undersecretary of defense for acquisition, technology, and logistics, Carter shepherded a series of controversial budget cuts.

Long before joining the Obama administration, however, Carter was seen as a national security whiz.

He arrived at the Pentagon in 1981 as a civilian program analyst, and three years later wrote a paper on missile defense that concluded that President Reagan’s prized proposal for a space-based shield — dubbed the “Star Wars” program — was unworkable, gaining him a reputation for taking on his superiors.

At the end of the Cold War, as assistant secretary of defense for international security policy, Carter “took the lead in working with the countries of Eastern Europe and former Soviet republics and bringing them into the Western security umbrella,” former secretary of defense William Perry, one of Carter’s mentors, recently recalled in an interview.

Carter, who never served in the military, is not considered a dove. In a controversial 2006 article he cowrote with Perry, he advocated bombing North Korea if it did not give up its nuclear weapons program.

Carter has supporters in both parties. At a 2012 hearing, Senator John McCain, Republican of Arizona, told him that “I have come to know you as a hard-working, honest, and committed public servant.”

That same year, in another interview with the Globe, Carter summarized what he sees as the Pentagon’s foremost challenge.

“We are trying to manage to a lower budget at a time when the threat is not receding,” he said. “The world hasn’t gotten any safer.”

Previous coverage:

2012: Cabinet nod could be next for Harvard’s Ashton Carter

2012: Pentagon’s second in charge oversees downsizing

Material from the Associated Press was used in this report. Bryan Bender can be reached at bryan.bender@globe.com.