WASHINGTON — As President Obama looks to fill Cabinet positions, the spotlight is steadily focusing on longtime Harvard physicist and leading national security strategist Ashton B. Carter, a top lieutenant to the past two secretaries of defense who has influential supporters among both parties in Congress.

According to several knowledgable officials, Carter, 58, has been on the short list to replace Secretary of Defense Leon Panetta when he steps down as planned next year, and his chances have improved with growing signs that another top candidate, former senator Chuck Hagel, a Republican, could face a difficult confirmation process in the Senate.