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 apologise.
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.
Senator Eastland (pictured in 1961 left with former AG Robert Kennedy) viewed black people as an "inferior race"
"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."
Mr Biden speaks in 2008 after being formally introduced as Barack Obama's running mate
"The point I'm making is you don't have to agree. You don't have to like the people in terms of their views," Mr Biden said. "But you just simply make the case and you beat them without changing the system."
Speaking to MSNBC, Biden advisor Anita Dunn said: "He didn't praise them, he didn't praise their positions, he certainly didn't endorse their positions.
"It's a story he's told many times. And the point of the story is that you have to be able to work with people, even if they hold positions repugnant to you, in order to make some progress."
.@JoeBiden did not praise a segregationist. That is a disingenuous take. He basically said sometimes in Congress, one has to work with terrible or down right racist folks to get things done. And then went on to say when you can't work with them, work around them.
— Symone D. Sanders (@SymoneDSanders) June 19, 2019
How are Democratic candidates responding?
California Senator Kamala Harris, who is black, lambasted Mr Biden for "coddling" segregationists.
She said the comments "suggests to me that he doesn't understand… the dark history of our country".
New Jersey Senator Cory Booker, who is also black, took offence at Mr Biden's call for him to apologise as "so insulting".
"He knows better," Mr Booker told CNN. "And at a time when Donald Trump never apologises for anything, I know Joe Biden. He's better than this."
New York City Mayor Bill de Blasio, took to Twitter to share a photo of his inter-racial family.
It”™s 2019 & @JoeBiden is longing for the good old days of “civility” typified by James Eastland. Eastland thought my multiracial family should be illegal & that whites were entitled to “the pursuit of dead n*ggers." (1/2) pic.twitter.com/yoOOkpaTX2
— Bill de Blasio (@BilldeBlasio) June 19, 2019
Mr Biden and nearly all of the Democratic candidates will be travelling to South Carolina this weekend where they will all be aggressively courting support from black communities.
The three-decade-old Fish Fry picnic – which the candidates will attend – is hosted by House of Representatives Majority Whip Jim Clyburn, who defended Mr Biden on Wednesday by recalling his own work with southern racists.
"You don't have to agree with people to work with them," said Mr Clyburn, the highest-ranking African-American in Congress.
Mr Biden has capitalised on his relationship as former deputy to president Barack Obama – the first-ever black US president, who is still revered among Democratic voters.
Mr Biden launched his presidential campaign in April by condemning Donald Trump's response to a racist march in Charlottesville, Virginia, in 2017.