A second factor may have been Wright, who, a week before the primary, reappeared for the first time since the controversial video was aired and repeated many of his most controversial views at the National Press Club. Obama, after a day of hesitation, denounced his former pastor and broke with him.

Conventional wisdom suggested that the second episode would crystallize opposition to Obama and help Clinton in North Carolina. In retrospect, Clinton advisers believe, it actually hardened Obama’s support in the black community…

On Monday night, the Clintons flew back to New York after a long day of campaigning in South Dakota, a state she would win on the day Obama clinched the nomination. During the plane ride home, they sat in the front row, and no one dared talk to them. One aide said the tension in the front cabin — visible to reporters in the back — was painful to endure.

Another member of the inner circle described Bill Clinton as coming “unhinged” in the final hours, raising his voice in phone calls with superdelegates, constantly revisiting what his wife’s options were for staying in the race. “He keeps asking me, ‘What about so-and-so? What about so-and-so?’ ” the supporter recalled, saying Clinton wanted constant updates on superdelegate movements.