Paul Krugman may be a terrific economist, but he should study his history. In a trenchant New York Times column titled “How Change Happens,” Dr. Krugman asserts that legislative change requires “hardheaded realism” and “accepting half loaves.” Dismissing presidential candidate Bernie Sanders’s uncompromising idealism as “happy dreams” and “destructive self-indulgence,” he asks rhetorically, “When has their theory of change ever worked? Even F.D.R., who rode the depths of the Great Depression to a huge majority, had to be politically pragmatic, working not just with special interest groups but also with Southern racists.”

But F.D.R. offers a poor parallel to the political situation today. Democrats dominated Congress during his 12-year tenure with supermajorities that sometimes surpassed 75 percent. Krugman is correct that F.D.R. compromised some elements of his agenda, but he compromised from a position of strength, and most of his landmark New Deal proposals passed with large majorities.