People who follow politics probably know that Bernie Sanders, the U.S. senator from Vermont who is challenging Hillary Clinton for the Democratic presidential nomination, is a socialist. Whether they give a fig is a separate matter, which may tell you something about today’s Democrats. Mr. Sanders is currently drawing the largest crowds of any candidate in either party. On Monday, he drew a crowd his campaign estimated at 27,500 to the Los Angeles Memorial Sports Arena, including those in an overflow area outside, watching on giant video screens. Over the weekend, 28,000 people turned out to see him in Portland,...