The single moment that made postwar liberalism feel most like a cause worth fighting for came in the darkness of April 4, 1968, when an Indianapolis crowd, assembled to hear Robert F. Kennedy campaign for the Democratic presidential nomination, instead met a man obliged to tell them that Martin Luther King Jr. had been murdered. When Kennedy broke the news, a desperate wail burst from the throats of those gathered, a sound unlike any other, bespeaking the tide of anguish and anger about to rush over the republic, sweeping reason before it—but not yet, or not here, not if Kennedy had his way.

Speaking off the cuff, he claimed a shared sorrow—his own brother had been killed in the line of political duty, at a time when he had begun to align himself with King. Thinking of what he had learned from the violence, Kennedy recited from Aeschylus the lines that had given him leave to accept that he would never forget or stop feeling pain but that he could nevertheless carry the cause forward. In the wake of this new killing Americans could, Kennedy said, divide themselves from their fellows—but that was not what the country needed. “What we need in the United States,” he said, was “love and wisdom, and compassion toward one another, and a feeling of justice toward those who still suffer within our country, whether they be white or whether they be black.” And the crowd that had begun listening in grief and despair now applauded, and unusually among American cities, Indianapolis did not see violence that night....

In that peak moment of liberalism, one could without embarrassment invoke love as, indeed, all you need; love would do everything that pop music promised, carry you through the darkness and bind you together with all the lonely souls in the nation’s night, tiding you over until the dawn. Certainly there was no other vocabulary, no logic of self-interest or language of patriotism, that seemed able to transcend the divisions among Americans and induce them to support policies for the benefit of others—to do for their country, rather than for themselves. Love gave liberalism, and liberals, guts.

And yet liberals often—and at last completely—rejected it, succumbing to a terrible impulse toward mere rationality. Eric Alterman and Kevin Mattson, in their excellent history of postwar American liberalism, The Cause, circle back occasionally to Lionel Trilling’s Liberal Imagination, with its warning that liberalism “drifts toward a denial of the emotions and the imagination,” becoming “mechanical”—or just dead. Michael Kazin, in American Dreamers, his history of American leftists, suggests that it was the radicals—now all but vanished except as bogeymen—that helped give liberalism life. Each book is a superb history that shows what master historians at the peak of their powers and knowledge can do. Each provides opportunities to rethink the American political tradition.