President Trump’s approval rating among blacks stood at 40% in a Rasmussen poll two days before Halloween. Rasmussen surveys are often friendlier to the president than those of other polling organizations, so critics are quick to dismiss them as outliers. In this instance, however, that could be a mistake.

Black voters are critical to Democratic prospects in 2020. Two years ago, blacks did not turn out in the numbers Hillary Clinton was expecting, and people in heavily black neighborhoods who did cast a ballot voted more Republican...