The transformation of the GOP into the party of Patrick J. Buchanan and Donald J. Trump—defined by cultural resentments, crude populism, and ethnic nationalism—is among the most important political stories of this century. But the GOP is hardly the only party that is undergoing some alarming tectonic shifts. Liberals wondering why conservatives who worry about Trump don’t join the Democrats should consider what is happening on their own side of the aisle. If you want to understand just how radicalized the Democratic Party has become in recent years, look at the ascent of Senator Bernie Sanders of Vermont. A self-proclaimed...