Democrats faced a stark choice in 2016: democratic socialist Bernie Sanders or corporate centrist Hillary Clinton. There was only one choice for progressives, who filled stadiums, caucus sites and not quite enough voting booths for Mr. Sanders.

This time around, the options seem endless. Progressives reveled in Betomania until he turned out to be an empty Banana Republic shirt. They flirted with Kamala Harris because she was mean to Brett Kavanaugh. Then they found out she was mean to everyone, especially innocent defendants....