Shortly before President Obama left office, a black friend remarked to me over lunch that we might not see another black president elected for a very long time. He said white Americans had taken a chance on Mr. Obama and that he not only underperformed but racialized the presidency in ways that many voters didn’t expect.

I’m less pessimistic about the prospect of another black president, but the departure of Sens. Cory Booker of New Jersey and Kamala Harris of California from the 2020 race got me thinking about how they had...