WASHINGTON — James R. Schlesinger, a hawkish Republican who straddled the partisan divide to serve in Cabinet-level posts under three presidents, has died, a Washington think tank said today. He was 85.

The Center for Strategic and International Studies, where Schlesinger was a trustee, confirmed his death.

The onetime University of Virginia economics professor built an impressive national-security resume as defense secretary for Republican presidents Richard Nixon and Gerald Ford and was the nation’s first energy secretary under Democratic President Jimmy Carter. Earlier he was a top White House budget official, chairman of the Atomic Energy Commission and director of the Central Intelligence Agency — all under Nixon.

In later years, he served on a host of defense and energy-related task-forces and advisory committees and continued to push for more sophisticated nuclear weapons systems. He was a longtime member of the Pentagon’s Defense Policy Board and was appointed by President George W. Bush to the Homeland Security Advisory Committee.

The Harvard-educated Schlesinger gained a reputation as a perceptive thinker on nuclear strategy.