NEWPORT, Del. — In July 1974, with a federal court in Delaware on the verge of ordering busing to integrate Wilmington’s overwhelmingly black public schools, Joseph R. Biden Jr. arrived at a school auditorium in this predominantly white suburb to find himself the target of a political ambush.

Just two years after narrowly winning a Senate seat at the age of 29, Mr. Biden had recently cast two votes to protect the practice of busing to achieve desegregation — despite his own very public unease with it. He thought he had come to Newport simply to address a local civics organization. But when he got there, more than 200 people, organized by a largely white parent group that opposed busing, jeered and heckled Mr. Biden, demanding that he more vocally join their cause.

“If you think I’m in trouble with you people,” he said then, seeking to assure the crowd that he was on their side, “you ought to hear what my liberal friends are telling me.”

The meeting marked a turning point for the young senator, who counted himself a liberal Democrat and an ardent defender of civil rights. Not long after that verbal drubbing, Mr. Biden plunged headfirst into one of the most politically fraught and racially divisive topics in America. He emerged as the Democratic Party’s leading anti-busing crusader — a position that put him in league with Southern segregationists, at odds with liberal Republicans and helped change the dynamic of the Senate, turning even some leaders in his own party against busing as a desegregation tool.