It’s no exaggeration to say that American politics has operated in Herbert Croly’s and FDR’s shadow ever since the Great Depression—nearly a century. What’s striking about this election cycle’s Democratic presidential primary debates is how much they’ve been absolutely dominated by a man who’s been dead for 90 years. That man is Herbert Croly, the intellectual godfather of the progressive movement and the first editor of The New Republic. Croly’s most influential work, The Promise of American Life (which would have been even more wildly influential had Croly only had a halfway decent editor to cut down his meandering, ponderous...