We’re still a year from the 2020 presidential primaries, but Senator Kamala Harris is already showing America how far the Democratic Party has strayed from its roots.

Democrats rightly take pride in the 1928 nomination of Gov. Al Smith, which signaled the party’s affirmation that Catholics had a place in American public life. Ditto for Jack Kennedy, who told a gathering of skeptical Protestant ministers in 1960 that “if this election is decided on the basis that 40 million Americans lost their chance of being president on...