". . . if only Rudolf Hilferding and his ilk had been less blinkered ideologues who sought truth from old books rather than new facts, the SPD could have led a German 'New Deal' that would have been as great a success as Roosevelt's New Deal in America."
DeLong is talking about how the German Social Democrats circa 1931 stood around in policy paralysis as the Nazis and Communists siphoned off their support with programs of public works to overcome the effects of the Great Depression.
DeLong does not draw the implied parallel, I would:
The connection between this historical episode and the American Enterprise Institute and the Project for a New American Century is left as an exercise to the reader."
My own thoughts went to the present inability of liberal economists to fashion some reasonable policy in response to the downsides of globalization.