Democratic presidential candidate Joe Biden has refused to apologise for reminiscing on his work with two long-dead racist senators.
But amid a firestorm of criticism, the ex-US vice-president said he had "detested" the views of late senators who favoured separating the races.
His rivals have decried him for saying senators once "got things done" with "civility" with the segregationists.
It is the ugliest race so far in this Democratic race for the White House.
Mr Biden is still recovering from similar backlashes that led him to reverse course on federal funding for abortions and praising US Vice-President Mike Pence as a "decent" person.
The latest row began on Tuesday night when Mr Biden fondly recalled his working relationship after joining the Senate in the 1970s with two southern Democratic senators, Mississippi's James Eastland and Georgia's Senator Herman Talmadge.
Mr Biden, 76, said at a fundraiser in New York City that Talmadge had called him "son" but never "boy", referring to how racist whites addressed black men at the time.
But some of his rivals among more than 20 Democratic candidates in the 2020 field - including black senators Cory Booker and Kamala Harris - pounced on his remarks, demanding he apologises.
But the former US vice-president, who is currently leading opinion polls, pushed back while attending fundraisers in the Washington DC suburbs on Wednesday night.
"They know better," Mr Biden told reporters on the campaign trail. "Apologise for what? Cory should apologise. He knows better.
"There's not a racist bone in my body; I've been involved in civil rights my whole career. Period. Period. Period."
Mr Biden said he had "had to put up with" the segregationists, but that moderate Democrats "were able to beat them on everything they stood for".
He added: "We, in fact, detested what they stood for in terms of segregation and all the rest.