Mr. Biden, officials said, used his ties on Capitol Hill to champion another unpopular cause: Chuck Hagel, also an old Senate friend. With Mr. Hagel under attack over his positions on Iran and remarks he made about Israel, officials said Mr. Biden worked the phones to reassure skeptical senators and help eke out his confirmation as defense secretary.

Just as important to Mr. Biden’s rise is his role in policy debates. With the exception of the raid on Osama bin Laden, which Mr. Biden famously advised delaying, there are few big policy deliberations in which he has not lined up with Mr. Obama. On issues like Afghanistan or Syria, his cautious instincts have prevailed over those advocating a more aggressive course.

In addition to pushing for a faster timetable for withdrawing troops from Afghanistan, he was also among those who opposed supplying weapons to the rebels in Syria — a proposal developed by David H. Petraeus, the former director of the C.I.A., and supported by Hillary Rodham Clinton, then the secretary of state. Mr. Biden was joined in resisting the proposal by Thomas E. Donilon, the national security adviser, with whom he — not surprisingly — also has longstanding personal ties.