Joe Biden faced a growing backlash Wednesday from prominent Democrats - and a bit of second-guessing within his own campaign - over comments in which he proudly described his history of working hand in hand in the Senate with avowed racists.

Biden's remarks, which came at a fundraiser Tuesday night in which he said one segregationist senator "never called me 'boy,' he always called me 'son,' " seemed intended to highlight a central argument of his presidential candidacy: that he knows how to bring unity to a polarized nation.

Instead, they prompted another controversy for Biden's campaign - and the sharpest attacks yet from his rivals on matters of race that are central to his bid and important to black voters who are seen as a crucial force in deciding the Democratic nomination battle.

Sen. Cory Booker, D-N.J., whose parents faced racial discrimination when trying to move into a mostly white neighborhood in New Jersey, was explicit that Biden needed to apologize. Sen. Kamala Harris, D-Calif., said she was "deeply" concerned by Biden's remarks, telling reporters at the Capitol, "If those men had their way, I wouldn't be in the United States Senate and on this elevator right now."

"I am trying to figure out if I am more outraged or simply disappointed," said Donna Brazile, a longtime Democratic strategist who became the first black woman to run a presidential campaign when she managed Al Gore's 2000 White House bid. "He should apologize."

Biden on Wednesday evening pushed back aggressively against anyone calling on him to disavow his comments or apologize.

"They know better," he told reporters as he went to a fundraiser. "Apologize for what? Cory should apologize. 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."

Booker, during an interview Wednesday night on CNN, said that Biden calling on him to apologize was "really problematic" and that "I don't understand why he needs this lesson" about racial sensitivity.

"The fact that he has said something that an African American man could find very offensive and then to turn around and say, you know, 'I'm not a racist, you should apologize to me' . . . is so insulting and so missing the larger point that he should not have to have explained to him," Booker said. "He knows better. And at a time when Donald Trump never apologizes for anything . . . I know Joe Biden. He's better than this."

The eruption exposes a persistent disconnect in Biden's message on race as he seeks to establish primacy in the historically diverse Democratic presidential field ahead of the first debates next week. While he has cited President Donald Trump's racially divisive rhetoric as an inspiration for his candidacy and drawn strong early support from blacks, Biden, 76, has also struggled to explain his past views on issues of importance in the black community, such as criminal justice and school integration. Now, his history of collegiality with racists is being seen by many in his party as a reason to question his judgment - not, as Biden says, a sign of his civility.

As seemingly random as it was for Biden to reference Sen. James Eastland, a long-deceased segregationist senator from his own party, some in Biden's campaign had heard him discuss this relationship before - and warned him against mentioning it in public. Eastland, who represented Mississippi in the Senate from the early 1940s to 1978, often said that blacks were "an inferior race."

Aides said they had urged Biden to find a less toxic example.

"It might move him to pick a different senator," said one adviser, speaking on the condition of anonymity to discuss internal deliberations. "But he's not someone you can go to and just say, 'You've been doing this x number of years and you can't do this anymore.' "

Another adviser, expressing confidence that the fallout from Biden's remarks could soon subside, nonetheless worried that Biden could inflame matters if he jokes about it the way he has sought to defuse other campaign controversies.

Biden's popularity among blacks, due in part to his eight-year partnership with the country's first black president, has so far been a defining characteristic of the primary, helping give him a wide lead in polls amid a crowded field that includes two prominent black senators, Harris and Booker.

Several prominent black politicians - including House Majority Whip James Clyburn, D-S.C., the highest-ranking black member of Congress - came to Biden's defense on Wednesday, saying he was being unfairly maligned for making a broader point about working with those who hold views that he disagrees with.

"Look, he is the front-runner, he probably didn't say it in the most articulate manner," said Rep. Cedric Richmond, D-La., who is co-chairman of Biden's campaign and the former chairman of the Congressional Black Caucus. "But I think the sentiment is something that we all know in the legislative body - that you work with people you don't agree with."

The debate over Biden's comments came in a week when race continued to hover over American politics. The House held a hearing Wednesday on slavery reparations, and a day earlier, white nationalists marched outside Trump's reelection rally. Trump also refused to apologize for his past stance that the Central Park Five, a group of black teenagers who were convicted on charges of raping a white woman in New York and later exonerated, deserved the death penalty.

Biden and almost all of his Democratic rivals are heading to South Carolina this weekend for events targeted at winning over that state's large black electorate, including an annual fish fry hosted by Clyburn.

Biden's remarks also offer a reminder of his yearslong history of verbal misfires and general lack of oratorical discipline, which are blamed in part for damaging his previous runs for the presidency. That history has prompted aides to tightly control his public appearances in recent weeks, often keeping reporters at bay and heightening the anticipation around how he would perform when he joins his rivals on the debate stage in Miami.

He has focused much of his time on fundraisers, and his campaign has allowed reporters into the events to cover his remarks.

"I was in a caucus with James O. Eastland. He never called me 'boy,' he always called me 'son,' " Biden said on Tuesday night, imitating the Southern drawl of the Mississippi senator. Biden also referenced Herman Talmadge, a senator from Georgia who supported segregated schools.

"You go down the list of all these guys. Well guess what? At least there was some civility," he said. "But today, you look at the other side and you're the enemy. Not the opposition, the enemy. We don't talk to each other anymore."

His campaign advisers said that what Biden meant was getting lost in part because of how Biden said it.

"He basically said sometimes in Congress, one has to work with terrible or downright racist folks to get things done," Symone Sanders, a senior adviser, wrote on Twitter. "And then went on to say when you can't work with them, work around them."

Others pointed to some of Biden's competitors, who have worked with Republicans who hold views that many Democrats don't agree with.

"Cory Booker, who has worked with Jeff Sessions on many things," Biden adviser Anita Dunn said on MSNBC. "Elizabeth Warren talks about how she's worked with Chuck Grassley, who led the fight for Brett Kavanaugh and who wouldn't even meet with Merrick Garland."

On Wednesday night, Biden mounted a further defense and cited his longtime work on voting rights legislation. "I could not have disagreed with Jim Eastland more in the sense he was a segregationist. I ran for the United States Senate because I disagreed with the views of the segregationists, many of them in the Senate at the time."

"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," he added. "But you just simply make the case and you beat them. You beat them without changing the system."

Many expressing alarm Wednesday at Biden's remarks said they were not just concerned about his views on collaborating with those with whom he disagrees but also his seemingly flippant reference to the word "boy" and how it was used by a racist.

"He's trying to make a point about civility, and he's suggesting that he was being civil. But he's also suggesting the segregationists are being civil," said Cornell William Brooks, a former president of the NAACP. "And the example he's using is one where they decline to call him the polite version of the N-word reserved for African American individuals."

For the second week in a row - last week, it was Biden's comments opposing the federal funding of abortions, a position he reversed - many in the Democratic primary field pointedly and personally criticized Biden.

"Vice President Biden's relationships with proud segregationists are not the model for how we make America a safer and more inclusive place for black people, and for everyone," Booker said in a statement. "And frankly, I'm disappointed that he hasn't issued an immediate apology for the pain his words are dredging up for many Americans. He should."

Sen. Bernie Sanders, I-Vt., who has often struggled to appeal to black voters, said he agreed with Booker, while Sen. Elizabeth Warren, D-Mass., told reporters, "I'm not here to criticize other Democrats, but it's never OK to celebrate segregationists. Never."

New York Mayor Bill de Blasio posted a response on Twitter that featured a photo of him with his wife, who is black, and their two children.

"Eastland thought my multiracial family should be illegal & that whites were entitled to 'the pursuit of dead n------,' " he wrote.

Several Democrats on Capitol Hill said Biden was simply reflecting the way things should work in a functioning Senate. Biden also got some support from Republicans who have worked with him.

"Let me say this about Joe Biden," said Sen. Lindsey Graham, R-S.C. "He has consistently crossed the aisle to try to find solutions to problems, working with people he disagreed with."

"He should be proud of that. Don't run away from it," said Graham.

Graham said he had not seen Biden's comments on the segregationist senators. When a reporter read him portions of the remarks, Graham responded, "See, I applaud that Joe Biden. I've always found him to a be a guy that wanted to get things done."

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The Washington Post's Scott Clement, Mike DeBonis and Cleve R. Wootson Jr. contributed to this report.