President Obama is expected to nominate Ashton Carter, a physicist with extensive experience in the Pentagon's leadership, as secretary of Defense to replace Chuck Hagel, according to administration officials.
If confirmed by the Senate, Carter will lead the Pentagon as it expands military operations against Islamic State militants in Iraq and Syria and deals with a resurgent Russia and renewed violence in Afghanistan. The military will also rebalance forces in Asia, even as it faces budget constraints and a cutback in troop levels.
Carter could prove more aggressive than the self-effacing Hagel in defending the administration's policies in public, and at pushing back in private against White House attempts to keep tight limits on military operations.
Hagel announced his resignation on Nov. 24 under pressure from the White House and amid disagreements between the Pentagon and the White House over the president's strategy in the latest Middle East war. He will stay in the job until his replacement is confirmed.
Carter, 60, never served in uniform. But the military establishment regarded him as a bold thinker when he served in the Obama administration as deputy Defense secretary, the No. 2 post, and as chief weapons buyer, the No. 3 post. He resigned a year ago.
He was confirmed by unanimous Senate votes for his two previous Pentagon posts and is unlikely to face strong opposition this time.
The White House did not dispute that Carter was the leading contender. But it was not prepared to announce the president's choice and was still deliberating on timing.
Among the factors playing into the decision is the vetting process, which was not yet complete, one senior administration official said, speaking on condition of anonymity because the decision is not yet final.
White House Press Secretary Josh Earnest declined to confirm that Obama had chosen Carter, but spoke highly of him at a briefing.
"He's somebody that certainly deserves and has demonstrated strong bipartisan support for his previous service in government," Earnest said. "He is somebody that does have a detailed understanding of the way that the Department of Defense works."
The Pentagon declined to comment. "This is a decision that only the president can make and only the president can announce," Pentagon spokesman Rear Adm. John Kirby told reporters.
Carter has close ties to many military commanders and key members of Congress, including Sen. John McCain (R-Ariz.), who is expected to lead the Senate Armed Services Committee when the Republican Party gains majority control of the chamber in January. That committee will consider the Pentagon nominee.
When Carter resigned as deputy Defense secretary last December, McCain said, "On many issues relating to defense and national security, Ash and I have had our differences. Some have been profound. But Ash has always conducted himself in a manner that appreciated the valid concerns underlying opposing views."
The lame-duck Congress is expected to recess in mid-December, so hearings are unlikely until the new Congress takes office. Loretta Lynch, Obama's choice to replace Atty. Gen. Eric H. Holder Jr., also faces confirmation hearings.
Carter holds degrees in physics and medieval history from Yale and a doctorate in theoretical physics from Oxford University, where he was a Rhodes scholar. He has taught for years at Harvard.
He first joined the Pentagon in 1981 under President Reagan as a technical analyst. A decade later, President Clinton named him assistant secretary of Defense for international security policy, where he worked to ensure that the former Soviet nuclear weapons stockpile did not fall into the hands of potential terrorists or rogue states.
Carter left the Pentagon in 1996. He returned in 2009 to serve as the chief weapons buyer, including the $400-billion F-35 Joint Strike Fighter program. He was named deputy secretary in 2011 but left again after Obama picked Hagel to succeed Leon E. Panetta.