After his Senate service and presidential races, Hart resumed his law practice. He remained moderately active in public policy matters, serving on the bipartisan US Commission on National Security/21st Century, also known as the Hart–Rudman Commission, commissioned on behalf of Bill Clinton in 1998 to study U.S. homeland security. The commission issued several findings calling for broad changes to security policy, but none were implemented until after the September 11, 2001 terrorist attacks. He earned a D.Phil. in politics from the University of Oxford in 2001 with a dissertation entitled "The Restoration of the Republic"; while at Oxford, he was a member of St Antony's College.