“Why so many alte kockers? Where is the rising generation?” The grumbler sitting behind me at the conference on “Jews and the Left,” sponsored by YIVO last week at the Center for Jewish History in New York, was not exactly being fair. Any academic conference will attract an older-skewing audience, and for all the gray hair in the seats and on the dais, the YIVO conference did have its share of eager young attendees.

Behind the complaint, however, it was possible to hear a larger, more painful question. For the first two-thirds of the 20th century, from the first immigrant generation through the baby boom, the radical and revolutionary left played a hugely important role in defining how the rest of America saw Jews and...