Democratic White House front-runner Joe Biden has come under ferocious attack for his record on race in a televised debate with nine rivals.
Senator Kamala Harris assailed him for touting his past work with racist 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 Donald Trump in next year’s election.
How did the flashpoint occur?
The clash of the night unfolded as the issue of racial profiling and policing was discussed during Thursday night’s forum in Miami, Florida.
Ms Harris – the only black woman in the crowded Democratic field of 20 – talked over the debate moderators to insist: “I would like to speak on the issue of race.”
Turning to Mr Biden she took him to task for citing his past work with fellow Democratic senators who favoured segregation of the races.
Ms Harris, the daughter of Jamaican and Indian immigrants, said to Mr Biden: “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 taking in a lead role in the mid-1970s in the fight against sending white children 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 hit back: “It’s a mischaracterization of my position across the board: I did not praise racists. That is not true.
He denied praising the racist senators and pointed out that he had been a criminal defence lawyer, in a veiled jab at Ms Harris, a public prosecutor.
She has been previously forced to defend her record to African-American voters as a San Francisco district attorney, amid claims she breached the rights of defendants and opposed criminal justice reforms.
On stage, Ms Harris challenged Mr Biden to disavow his prior opposition to bussing in America.
He insisted he was only opposed to it being ordered by the federal government but had no problem with the policy being implemented at a local level.