To outline his fears about the U.S. economy, Raghuram Rajan picked a tough crowd.

It was August 2005, at an annual gathering of high-powered economists at Jackson Hole, Wyo. -- and that year they were honoring Alan Greenspan. Mr. Greenspan, a giant of 20th-century economic policy, was about to retire as Federal Reserve chairman after presiding over a historic period of economic growth.

Mr....