The Democratic Party’s list of 2020 contenders grew by two last week when Sens. Kamala Harris and Cory Booker made their White House ambitions official. Liberals and the media are celebrating the “diversity” of the candidates thus far—which I suppose is something to cheer if your measure of diversity is skin deep.

Ms. Harris and Mr. Booker “identify as black,” to use the popular locution, and join a presidential field that already features candidates who identify as female, Hispanic and gay. Liberals tell us that they pine...