JEFF GREENFIELD, SPECIAL CORRESPONDENT:

What makes this "tribalism" particularly dangerous is that it has grown at a time when one of America's core convictions—the worth of a free society—has eroded, especially among the young. They are simply less and less convinced that democracy is all that important.

Among Americans born in the 1930s, 72% said that living in a democracy was "essential." Among those born in in the 1980s, the number is thirty percent. The less faith in an open society, the more reason there is to believe that politics is more like warfare than a contest for power where limits apply.

The guardrails that protect our constitutional republic have endured for more than two centuries, in the face of challenges far greater than today's. But when you combine a growing sense that your political opponents are enemies with doubts about the very worth of a free society, you threaten some of our bedrock assumptions about how the oldest representative democracy in the world stays healthy