Ever since Arthur M. Schlesinger Jr. used "imperial presidency" as a book title, the term has become central to the debate about the balance of power in the U.S. government. Since the presidency of George W. Bush, when advocates of executive power such as Dick Cheney gained ascendancy, the argument has blazed hotter than ever. Many argue the Constitution itself is in grave danger. What is to be done? The answer, according to legal scholars Eric Posner...