To make sense of President Trump’s dust-up with Rep. Elijah Cummings, Speaker Nancy Pelosi and the rest of the Democratic Party, you have to go back to Baltimore in April 1968, when the city was overwhelmed by a riot in the wake of Martin Luther King Jr.’s assassination. The National Guard and city police proved unable to contain the situation. Mayor Tommy D’Alesandro III, Mrs. Pelosi’s elder brother, pleaded with President Lyndon B. Johnson to send in federal troops.

In 1968 Maryland hadn’t yet been absorbed by the wealth...