One of the first to recognize this was Senate Majority Leader Mitch McConnell. “I can pretty safely say this is the first time in my political career that I thought the essence of America was being debated,” he tells me in an interview.

“I never thought we [would be] debating things that were debated in the ’30s, both communism and socialism,” Mr. McConnell says. Those ideologies were “largely discredited at a time when Americans could have found these arguments pretty appealing in the middle of the Great Depression.”

Mr. McConnell thinks socialism ought to be a tough sell today, given the prosperous economy and low unemployment. But maybe not. Polls show socialism has risen in stature, which Mr. McConnell sees as evidence of the new era. Last year, he notes, Gallup found for the first time that Democrats have a more “positive view” of socialism than of capitalism.