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 racial segregation.
His rivals have blasted him for saying working with "civility" with the segregationists had "got things done".
It is the ugliest row so far in this Democratic race for the White House.
How did Biden land in trouble again?
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 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.
"At least there was some civility," he said. "We got things done."
Some of his rivals who are vying to be the Democratic party nominee for the November 2020 presidential election, including Senator Cory Booker, pounced on his remarks.
But the former US vice-president, who is currently leading opinion polls, fought back on Wednesday night.
"They know better," Mr Biden told reporters while attending fundraisers in the Washington DC suburbs.
"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 of the segregationists: "We, in fact, detested what they stood for in terms of segregation and all the rest."
Biden advisor Anita Dunn told MSNBC: "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 he did not seem to understand "the dark history of our country".
New Jersey Senator Cory Booker, who is also black, said it was "so insulting" that Mr Biden had called on him to apologise.
"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 tweeted his criticism, along with a photo of his inter-racial family.
But other prominent black lawmakers, including the highest-ranking African-American in Congress, Jim Clyburn, defended Mr Biden.
"You don't have to agree with people to work with them," said Mr Clyburn, who is House of Representatives majority whip.
Speaker of the House of Representatives Nancy Pelosi also supported Mr Biden, saying it was important for candidates to convey authenticity, and "Joe Biden is authentic".
"He considers certain things a resource. That he has worked across the aisle. That's what he was saying." She added that the back-and-forth is "not what this election is about".
Nearly all of the Democratic candidates will travel to South Carolina this weekend to aggressively court support from black voters.
Mr Biden will be among those attending the traditional fish fry picnic, which is hosted by Mr Clyburn.
The former vice-president has capitalised on his time as deputy to President Barack Obama, the first-ever black US president who is still revered among Democrats.
Mr Biden launched his campaign in April by condemning President Donald Trump's response to a racist march in Charlottesville, Virginia, in 2017.