LOS ANGELES — Reaching into jubilant crowds from atop the back seat of a slow-moving convertible, walking the streets of riot-torn Watts, sitting with Cesar Chavez in the Central Valley — these are the frozen moments of Robert F. Kennedy’s campaign for president in California in 1968.

There is another one, of course, a final one: lying on the kitchen floor of the Ambassador Hotel in Los Angeles, a busboy cradling his head.

The date was June 5, 1968.

Days earlier, Mr. Kennedy had flown into the city to campaign in the California primary, his presidential hopes hanging in the balance. He had just lost to Eugene McCarthy in the primary in Oregon, the first election loss ever for a Kennedy. But Oregon was mostly white; in California, Mr. Kennedy touched his natural constituency — impoverished African-Americans and disenfranchised farmworkers from Mexico.

And so in California, the promise of his candidacy rested: to heal a nation torn by the Vietnam War and divided by race and class. A big win there, his aides hoped, could convince Mr. McCarthy to drop from the race and the power brokers in the Democratic Party to back Mr. Kennedy, clearing a path to the nomination at the convention later that summer in Chicago.