(CNN) It was the moment everyone was talking about after Thursday's Democratic presidential debate.
At the center of the heated exchange between Sen. Kamala Harris and former Vice President Joe Biden was the use of busing to desegregate American schools -- and Biden's opposition to it in the 1970s.
Here's what you need to know about the issue.
The 1954 US Supreme Court ruling in Brown v. Board of Education brought an end to legal racial segregation in schools. But because of demographic trends, white flight to the suburbs and discriminatory housing practices like redlining, many neighborhoods across the country remained segregated. Combined with how cities drew school district lines, that meant schools remained segregated, too.
Enter busing.
After the 1971 Supreme Court ruling in Swann v. Charlotte-Mecklenburg Board of Education, lower courts began mandating busing to effectively desegregate schools. Black students started taking school buses to majority-white schools and white students to majority-black schools, often in neighborhoods far from where they lived.
It became one of the most controversial topics in US politics at the time.
Before the Supreme Court mandated school desegregation, schools for African American children often had meager resources compared to those white children attended, which in turn created racial gaps in achievement and opportunities.
Supporters of busing argued that the practice was necessary to effectively integrate schools -- and help correct the damaging legacy of school segregation.
But opponents of busing, many of whom were also opponents of school desegregation, argued that children were being transported to unsafe neighborhoods and objected to the long commutes children experienced. Others, including Biden, said that busing forced schools to achieve racial quotas and did not achieve equal opportunity for students.
In 1974, the Supreme Court delivered a blow to school integration efforts through its ruling in Milliken v. Bradley. The court held that a mandatory plan to bus black students from Detroit across school districts to mainly white suburbs was unconstitutional, arguing that there wasn't enough evidence to prove the suburban school districts were deliberately engaging in segregation. The decision accelerated white flight to the suburbs and worsened the problem of segregation in urban schools.
In Boston, opposition to busing turned violent. After a federal judge ordered the city to desegregate its public schools through busing in 1974, white residents responded with fierce criticism. Some pelted school buses carrying African-American children with bricks and bottles, and angry protesters mobbed schools. The riots garnered national attention.
While busing did produce opportunities for many black students, many court-mandated plans to integrate schools decades ago are no longer in place.
Today, the broader aim of practices like busing -- the total integration of schools and leveling of the playing field for American children of all races -- remains largely unfulfilled. The effort also prompted many northern, blue-collar whites to abandon the Democratic Party whose leaders had supported busing.
Biden's fight against busing more than four decades ago, as well as his praise of segregationist senators, has subjected him to scrutiny against the backdrop of today's more progressive Democratic Party.
The former vice president recently made headlines over his comments on working with two segregationist senators in the 1970s. During a fundraising event in New York, Biden pointed to his work with Mississippi Sen. James Eastland and Georgia Sen. Herman Talmadge -- both of whom opposed civil rights and desegregation -- as an example of how he could work with those he disagreed with.
"I was in a caucus with James O. Eastland. He never called me 'boy.' He always called me 'son,' " Biden told donors.
"Well, guess what? At least there was some civility. We got things done," he added. "We didn't agree on much of anything. We got things done. We got it finished."
Biden's comments drew sharp criticism from his 2020 Democratic presidential rivals, and Harris attacked him for the remarks during the first Democratic debate.
"It was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country. It was not only that, but you also worked with them to oppose busing," Harris said.
"And you know, there was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day. And that little girl was me," she added.
Biden called Harris' comments "a mischaracterization of my position across the board." He said that he opposed busing mandated by the Department of Education but that he took no issue with local districts busing as they saw fit.