One of the fascinating things about Richard Aldous’s book about historian-partisan Arthur M. Schlesinger Jr. is its composition. Schlesinger lived to be 89, dying an enviable death of a heart attack in a Manhattan steakhouse in 2007, but only a very few of the 389 pages in Aldous’s book concern the 39 years between 1968 and Schlesinger’s death.

That is the year the party of the midcentury Democrats fell victim to murder — in the killings of Martin Luther King Jr. and of Robert F. Kennedy — but also to suicide, in the rise of the forces of pacifism, moral equivalence, and identity politics that sadly control it today.