On Mar. 31, 1968, amid growing opposition to his Vietnam War policies, Lyndon Johnson announced that he would neither seek nor accept the Democratic Party’s presidential nomination. In the following months, Robert F. Kennedy, a leading contender to replace LBJ, was assassinated, and antiwar protests intensified.

In August, at a chaotic and contentious convention in Chicago, accompanied by violent clashes between police and protestors outside the hall, the delegates, most of whom were chosen by party leaders allied to Johnson, nominated Vice President Hubert Humphrey as their standard bearer.

As historian Allen Matusow revealed in his book “The Unraveling of America,” Humphrey, who had been banished from the White House inner circle because he privately opposed escalation of the war, recognized that if he deviated from the administration’s Vietnam policy, he risked losing Johnson’s support. At convention, LBJ had shot down a proposal designed to appease the party’s peace wing that would have recommended an unconditional halt to the bombing of North Vietnam.

“I should have stood my ground,” Humphrey wrote years later.

Humphrey, way behind Republican candidate Richard Nixon in the polls, delivered a nationally televised address on September 30 in which he supported a cessation of the bombing “as an acceptable risk for peace.” Although Humphrey had barely strayed from the administration’s position, Matusow reveals that this “Delphic utterance” gave antiwar liberals a signal about the........

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