By HELENE COOPER and MARK LANDLER

President Obama has selected Ashton B. Carter to be the next defense secretary, senior administration officials said on Tuesday, elevating a physicist and the Pentagon’s former chief weapons buyer to succeed Chuck Hagel, who was ousted last week.

As a former deputy defense secretary with a long history at the Pentagon, Mr. Carter helped accelerate the production and shipment of weaponry and armored vehicles to protect American troops from roadside bombs during the wars in Iraq and Afghanistan. Now, as the American military is building up again in Iraq, this time to fight Sunni militants with the Islamic State, Mr. Carter must manage the war effort as well as the intense budget pressures on the Pentagon in the face of mandatory spending cuts.

Mr. Carter’s formal nomination is expected in the next few days once the White House completes the vetting process, aides said. He is the only one of several top prospects who did not take himself out of the running for the job.

“It’s how much grief you want,” said Anthony H. Cordesman, a national security expert with the Center for Strategic and International Studies. “There’s a confirmation process where anyone with the political profile which the White House wants would run into a buzz saw in the Senate.”

But the nomination of Mr. Carter, 60, may be as close as the president can get to a confirmation process that will not create an uproar among Republicans. A number of Republican senators said on Tuesday that they would support Mr. Carter. Senator James M. Inhofe of Oklahoma, the ranking Republican on the Senate Armed Services Committee, said he foresaw no opposition to him.

Senator John McCain, Republican of Arizona, who is expected to be the committee’s next chairman, last year credited Mr. Carter’s “insatiable intellectual curiosity” during a tribute to him on the Senate floor.

Mr. Carter has degrees in physics and medieval history from Yale and a doctorate in theoretical physics from Oxford; he was a Rhodes Scholar, was a longtime member of the Harvard faculty and now lectures at Stanford. His senior thesis at Yale was on the use of Latin by monastic writers to describe the world of 12th-century Flanders.
Mr. Carter grew up in Philadelphia, where his first job was at a carwash at the age of 11. Soon fired for “wise-mouthing” the owner, according to an autobiographical sketch he wrote while at Harvard, Mr. Carter went on to work as a hospital orderly, a mate on a fishing boat and a counselor on a suicide prevention hotline.

Mr. Carter’s specialty as an academic was the control of nuclear weapons, and in posts in and out of government he devoted much of his time to thinking about how to control their numbers and how to prepare the United States if they were used. In 1994, under Defense Secretary William J. Perry, Mr. Carter was a central player in a nuclear crisis with North Korea, when the country threw out international inspectors and began what turned out to be a long, and successful, race to develop nuclear weapons.
At the Pentagon, Mr. Carter served from 2011 to 2013 as deputy defense secretary, the No. 2 position, in which he managed a $600 billion annual budget, more than two million uniformed and civilian employees and the beginning of what is scheduled to be a decade of $500 billion in budget cuts. From 2009 to 2011, he bought weapons and scaled back or canceled outdated programs as the Pentagon’s under secretary for acquisition, technology and logistics.

In 2013, Mr. Carter was a candidate to succeed Leon E. Panetta as defense secretary, but he was passed over for Mr. Hagel, a Republican who was a former senator from Nebraska and who was seen as the right person to manage what the White House expected to be the end of the wars in Iraq and Afghanistan. Mr. Obama asked Mr. Carter to stay for a year and manage the Pentagon under Mr. Hagel, which he did, although the arrangement was considered awkward.

By late this year, White House officials viewed Mr. Hagel as too passive in the face of rising threats overseas. He resigned under pressure.

Mr. Carter could not be reached for comment on Tuesday.

At the Pentagon, Mr. Carter pushed a program to get mine-resistant vehicles into the field to protect troops in Iraq and Afghanistan, then presided over the end of the program when the administration started to pull troops from both countries. His office at the Pentagon was dotted with pictures of him in the field — in those mine-resistant vehicles — and at a hospital visiting a soldier who had been grievously wounded when a mine exploded.

On that visit, the soldier asked to be reunited with his bomb-sniffing dog. Mr. Carter started a hunt through the military to find the dog. “You have no idea,” he said one afternoon in his office, “how much paperwork you need to go through to get a canine out of the military.”

The dog now lives with its wartime partner.

At the Pentagon, both military personnel and civilians greeted the news of Mr. Carter’s pending nomination well, and several officials expressed relief that the Pentagon would be getting a defense secretary with a minimal learning curve.

But Mr. Carter, who never served in the military, may have the same problems as Mr. Hagel penetrating the inner White House national security circle, where power is concentrated among a handful of longtime Obama aides. Mr. Carter would also come into the job without the benefit of a close personal relationship with the president.

But he has worked to develop relations with the White House, former colleagues said, and in particular has cultivated Denis R. McDonough, Mr. Obama’s chief of staff, even visiting wounded service members with him.

“This will not be Ash’s first time at that table,” said Jeremy B. Bash, who is a former chief of staff to Mr. Panetta and is advising Mr. Carter on his nomination.

The candidates who withdrew from consideration for the Pentagon job include Senator Jack Reed, a Rhode Island Democrat, and Michèle A. Flournoy, a former under secretary of defense, who cited family concerns. Jeh Johnson, the secretary of Homeland Security, was also mentioned as a candidate, but he said he was happy in his job.

Selecting Mr. Johnson, who as the general counsel at the Pentagon in Mr. Obama’s first term led the repeal of the “don’t ask, don’t tell” policy that kept gay men and lesbians from serving openly in the military, would have set up not one but two potentially bloody confirmation fights.

Mr. Johnson is a contentious figure among Mr. Obama’s supporters on the left because he was one of the legal architects of the administration’s policy on targeted killings using drone strikes. And Republicans, who are angry about Mr. Obama’s executive actions shielding five million undocumented immigrants from deportation, might have tried to hold up a Homeland Security nominee until the immigration issue was resolved to their liking.