Democratic White House front-runner Joe Biden has come under fierce attack for his record on race in a televised debate with nine rivals.
Senator Kamala Harris assailed him for touting his past work with bigoted senators and having once opposed a policy to foster diversity in schools.
He said she had “mischaracterised” his position, insisting he had entered politics to champion civil rights.
The candidates are vying to take on Republican President Donald Trump.
Fireworks also flew in the first debate with 10 other Democrats on Wednesday.
The top 20 contenders will be winnowed until a winner is picked at the party convention next summer, before he or she faces Mr Trump in the November 2020 election.
How did Biden v Harris unfold?
The fight of the night erupted midway through Thursday’s second forum in Miami, Florida.
Ms Harris – the only black woman in the Democratic field – pilloried Mr Biden for this month having cited his work decades ago in the Senate with fellow Democrats who favoured segregation of the races.
Turning to him, she said: “I do not believe you are a racist and I agree with you when you commit yourself to the importance of finding common ground.
“But I also believe, and it’s personal and I was actually very, 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.”
She also took him to task for his political advocacy in the mid-1970s against sending white children by bus to majority-black schools in other neighbourhoods, and vice versa.
“And it was not only that,” the 54-year-old said, “but you also worked with them [racist senators] to oppose bussing.
“And there was a little girl in California who was part of the second class to integrate her public school and she was bussed to school every day. And that little girl was me.”
Mr Biden bristled: “It’s a mischaracterisation of my position across the board. I did not praise racists. That is not true.”
He said last week he “detested” the segregationists’ views.
Ms Harris challenged Mr Biden to disavow his past opposition to bussing.
He insisted he was only against the policy being mandated by the federal government, but had no problem with its implementation at a state level.
Mr Biden brought up his two terms as vice-president to Barack Obama, America’s first black commander-in-chief.