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.