Democratic White House front-runner Joe Biden came under fierce attack for his record on race in a televised debate with nine rivals on Thursday.
Senator Kamala Harris raised his past work with bigoted senators, and his previous opposition to a policy combating segregation 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 contenders on Wednesday.
The Democratic candidate will be chosen at the party's 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 halfway through Thursday's debate in Miami, Florida.
Ms Harris - the only black woman in the Democratic field - pilloried Mr Biden for having recently reminisced about working with two Democratic senators who favoured racial segregation.
Turning to him, she said she did not believe he was a racist, but added: "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 working "with them [racist senators] to oppose bussing" - a policy of driving white children by bus to majority-black schools and vice versa, in the mid-1970s.
The policy aimed to undo the negative effects of Jim Crow-era racial segregation. Segregation of public schools was outlawed in 1954, but the racial inequality it fostered persisted.
"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," she said. "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, following a backlash.
He also said he was only against bussing being mandated by the federal government, but had no problem with it at state level.
Mr Biden then brought up his two terms as vice-president to Barack Obama, America's first black president.
Who will take on Trump?
Who else was on stage?
Debate watchers had been expecting a duel between the two front-runners - Mr Biden, a centrist, and Democratic socialist Bernie Sanders.
Mr Sanders, the Vermont senator who ran against Hilary Clinton to be the party's candidate in 2016, dominated the first half hour by defending his plans for universal healthcare, and called Mr Trump "a pathological liar and a racist".
Later, Pete Buttigieg, the 37-year-old mayor of South Bend, Indiana, faced questions about the fatal shooting of a black man by a white police officer in his home city.
Initially considered a rising star of the race, he conceded at the debate that he had failed to diversify South Bend's police force.
The other six on stage have all been polling at one per cent or less: Senators Kirsten Gillibrand and Michael Bennet, former Colorado Governor John Hickenlooper, congressman Eric Swalwell, self-help guru Marianne Williamson and entrepreneur Andrew Yang.
Mr Trump - who is at a G20 summit in Japan - pounced on his would-be challengers' unanimous support for providing healthcare coverage to undocumented immigrants.
All Democrats just raised their hands for giving millions of illegal aliens unlimited healthcare. How about taking care of American Citizens first!? That’s the end of that race!— Donald J. Trump (@realDonaldTrump) June 28, 2019
Where did it all go wrong for Biden?
Mr Biden, 76, was also confronted on an issue he presents as one of his strengths - political longevity.
Mr Swalwell said: "I was six years old when a presidential candidate came to the California Democratic convention and said, 'It's time to pass the torch to a new generation of Americans.'
"That candidate was then-Senator Joe Biden. Joe Biden was right when he said it was time to pass the torch to a new generation of Americans 32 years ago - he's still right today."
Mr Biden, who would be the oldest president ever elected, retorted: "I'm still holding on to that torch."
He has also faced criticism for flip-flopping on abortion rights, and for calling Vice-President Mike Pence "a decent guy".
Where did it all go right for Harris?
Ms Harris's confrontation with Mr Biden was not her only standout moment.
As the debate unravelled into a free-for-all shouting match, she was cheered for saying: "OK guys, America does not want to witness a food fight - they want to know how we're going to put food on their table!"
At another point she said, emphasising the female pronoun: "I will ensure that this microphone - that the president of the United States holds in her hand - is used in a way that is about reflecting the values of our country."
Ms Harris is not without her own political baggage, however.
She has had to defend her record as a San Francisco district attorney, amid claims she breached defendants' rights and opposed criminal justice reforms.
How did the first debate go?
Elizabeth Warren has been praised for her performance in Wednesday's debate, which was also in Miami.
The Massachusetts senator, who has pledged to institute a wealth tax and break up tech giants, described income inequality in the US as "corruption, pure and simple".
Meanwhile, several lesser-known contenders fired at each other - united in opposition to Mr Trump, but torn on whether the next president should put the nation on a more left-wing course.