One of this year’s dominant news stories is how Donald Trump has roiled the GOP. Yet Democrats face an internal rupture that may be larger and more durable. That is my takeaway after reading “The Split,” the New Republic’s extraordinary June 14 symposium of liberal activists, historians, intellectuals, political scientists, pollsters and writers.

“The schism between Hillary Clinton and Bernie Sanders is knit into the DNA of the modern Democratic Party,” writes historian Rick Perlstein. In 1924, he recounts, Klan supporters...