Few presidents have entered office under more difficult circumstances than Lyndon B. Johnson (1908–1973; served 1963–69). Johnson had been elected as vice president along with President John F. Kennedy in 1960, and he had served for three years with the customary lack of public attention endured by American vice presidents. Then, on a fateful visit to Dallas, Texas, on November 22, 1963, American political life changed. An assassin's bullets killed President Kennedy, and in the unsetting hours after the shooting, Johnson was sworn in as the president. When he appeared on national television that evening, he greeted a nation in deep grief over the death of one of the most beloved presidents in American history.

In some ways, Kennedy's shoes would be hard to fill. Few politicians in the twentieth century matched Kennedy's personal appeal and charm, and few...