Joseph R. Biden Jr. on Wednesday lashed out at his Democratic rivals who had condemned his fond recollections of working relationships with segregationists in the Senate, declining to apologize and defending his record on civil rights. The angry exchange shattered, at least for now, the relative comity that had marked the Democratic presidential primary.
Until Wednesday, many of the Democratic candidates had largely taken oblique swipes at Mr. Biden, while the former vice president sought to stay above the fray, training his sights on President Trump instead.
But a day after he invoked the 1970s, an era when he said he could find common ground with other senators — even virulent segregationists — his opponents offered their sharpest criticism yet.
Senator Kamala Harris of California said the former vice president “doesn’t understand the history of our country and the dark history of our country,” and Senator Cory Booker of New Jersey said Mr. Biden should immediately apologize for using segregationists to make a point about civility in the Senate.
Ms. Harris and Mr. Booker, who are both black, were not alone: Other candidates including Senators Elizabeth Warren of Massachusetts and Bernie Sanders of Vermont also weighed in with criticism. And even some of Mr. Biden’s senior campaign advisers were privately shaken by his remarks.
Yet for much of the day, Mr. Biden and his campaign appeared publicly unbowed and intent on defending, or at least explaining, his worldview of politics, which is rooted in his early days in the Senate when, he said, legislators who disagreed still worked together. He cited two defenders of segregation, Senators James O. Eastland of Mississippi and Herman E. Talmadge of Georgia, to make that point.
“Apologize for what?” he said Wednesday evening before appearing at a fund-raiser in Maryland, adding that he “could not have disagreed with Jim Eastland more.”
Asked by reporters about Mr. Booker’s demand that he apologize for his remarks, Mr. Biden said: “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.”
At another fund-raiser later that evening, Mr. Biden was sharper in his criticism of the two former Southern senators.
“We had to put up with the likes of like Jim Eastland and Hermy Talmadge and all those segregationists and all of that,” he said. “And the fact of the matter is that we were able to do it because we were able to win — we were able to beat them on everything they stood for.”
“We in fact detested what they stood for in terms of segregation and all the rest,” he continued.
Mr. Biden, a longtime supporter of the Voting Rights Act who has cited the civil rights movement as motivation for getting into politics, has many African-American allies, and on Wednesday a number of prominent black leaders defended him, including James E. Clyburn of South Carolina, the House Democratic whip and the highest-ranking African-American in Congress.
But for many Democrats, Mr. Biden’s decision to highlight those relationships seemed deeply misguided and out of touch with political change in America.
“I just really don’t understand for the life of me what the vice president could have been thinking, to bring the names of Mr. Talmadge and the others who are well-known conservative segregationists into any conversation referencing civility,” said Leah Daughtry, a veteran Democratic strategist who ran the 2008 and 2016 Democratic National Conventions and is African-American. She added, “He needs to issue an apology immediately.”
Mr. Biden, who is running for president in part on a message of national unity and reaching out to those with different viewpoints, particularly courted Mr. Eastland, in spite of his racist views and remarks.
The two men developed an “unlikely relationship,” as Mr. Biden put it in his 2007 book, as Mr. Eastland helped Mr. Biden achieve his first seat of power on the Senate Judiciary Committee. Mr. Biden and Mr. Eastland sharply disagreed on several civil rights-related matters, but they were also convenient allies, as both were vocal opponents of school integration through busing, a controversial topic at the time.
According to archives of The News Journal, the main newspaper serving Mr. Biden’s hometown, Wilmington, Del., he would also go on to present himself as a go-between on the Judiciary Committee for conservatives like Mr. Eastland and liberals like Edward M. Kennedy of Massachusetts, and to say that his criminal justice positions are “equidistant between the two factions.”
For Mr. Biden, the early Democratic front-runner, his remarks about Mr. Eastland and Mr. Talmadge — and the pointed criticism he drew on Wednesday — are a sharp example of how much Mr. Biden’s long record can be as much baggage as résumé.
Far from running away from his past, he has, in his seven-week-old campaign, proactively brought up his ties to multiple politicians who had records of opposing the civil rights movement, from Mr. Eastland and Mr. Talmadge to Senator Jesse Helms, the North Carolina arch-conservative. Mr. Biden routinely characterizes that era and those relationships as more civil and functional, with less demonization of the other side, even amid vigorous clashes over explosive issues like race.
Mr. Biden faces an unusual political dynamic: He is seeking to run against President Trump, who has his own heavy baggage and record of divisive positions and remarks. Yet Mr. Trump’s supporters have appeared willing to forgive or look past the president’s political troubles. Mr. Biden may be leading in the polls, but he faces a Democratic electorate that is far from sold on his candidacy and may not be willing to shrug off ghosts from his past — especially when Mr. Biden himself keeps bringing them up.
“He’s already looking ahead to the general election,” said Adrianne Shropshire, the executive director of BlackPAC, a political advocacy group. “White swing voters may be persuaded by his ability to work across the aisle with segregationists. But that’s not an argument that’s going to work for black Democratic primary voters.”
The remarks also raised questions about Mr. Biden’s political instincts as even allies privately said he could have made a similar argument about his amicable dealings with people who held opposing views without extolling his relationship with notorious segregationists.
One of the Biden campaign’s operating assumptions is that Democratic voters are more interested in bipartisanship than the loudest progressive voices on Twitter would suggest — but some allies implied Wednesday that Mr. Biden’s references overshadowed the broader point about civility he was trying to make.
Some pointed to moderate Republicans like the former Senators Bob Dole of Kansas or John McCain of Arizona — for whom Mr. Biden delivered an emotional eulogy last summer — as better names to mention, a discussion that came up internally.
In a separate sign of possible turbulence for Mr. Biden’s candidacy, his campaign experienced the first departure of a prominent political consultant. Mark Putnam, a high-profile Democratic strategist and producer of television ads, confirmed in a brief phone call that he had recently left Mr. Biden’s campaign.
Mr. Putnam declined to address the reasons for his departure, though they did not appear to be related to Mr. Biden’s struggles over the last few weeks concerning abortion rights and race.
“I wish the vice president well,” Mr. Putnam said.
Several advisers to Mr. Biden made a concerted public effort to explain and justify his remarks, saying that he was not praising segregationists but rather making a point about working with people with whom one disagrees.
But a number of Mr. Biden’s advisers used language about Mr. Eastland and Mr. Talmadge that was far harsher than Mr. Biden’s remarks on Tuesday.
Anita Dunn, a veteran Democratic operative, appeared on MSNBC on Wednesday afternoon to cast the segregationist senators as men Mr. Biden had long disagreed with on fundamental issues. She noted in particular Mr. Biden’s clashes with Mr. Eastland over the Voting Rights Act. Despite those differences, she argued, “There was civility involved.”
But she did not walk back his references to the two senators, which, as she said, Mr. Biden has made for years.
Mr. Biden had a similarly mixed relationship with Strom Thurmond, a South Carolina Republican with a history of racist views who was a key figure on the Senate Judiciary Committee in the 1980s and 1990s.
Though Mr. Biden and Mr. Thurmond had several disagreements over civil rights, they worked closely on crime legislation in the 1980s and 1990s. In 1997, Mr. Biden effusively praised him, including glowing words for Thurmond’s life before public office.
“Long before he was a committee chairman; indeed long before he came to the Senate so many years ago, Strom Thurmond was the consummate public servant,” Mr. Biden said.
Mr. Biden punctuated his speech with a joke: “Though he holds the record for the Senate’s longest filibuster, Strom Thurmond is a doer rather than a talker.”
He left out what Mr. Thurmond was filibustering when he set the record: the Civil Rights Act of 1957, which established the Department of Justice’s Civil Rights Division.
A number of prominent African-American lawmakers and community leaders came to Mr. Biden’s defense on Wednesday.
“I don’t see anything different in what Biden said to what we all do over here,” Mr. Clyburn said. “He didn’t say anything more than I would say to describe my work with Strom Thurmond and a few others.”
Mr. Clyburn, who participated in civil rights demonstrations in the 1960s and 1970s, said that Democrats of his generation needed to develop working relationships with segregationist Southern politicians like Thurmond, his state’s longtime senator.
The Rev. Joseph Darby, an influential African-American pastor from South Carolina — a state where polls show Mr. Biden with a commanding lead, currently, among black voters — dismissed the notion that Mr. Biden should apologize.
“People look at his overall record rather than cherry-picking some of the things he says,” said Mr. Darby, a longtime ally of Mr. Biden’s who also spoke positively about Ms. Harris and Ms. Warren. “They weren’t the examples I would use, but I don’t think that merits an apology. He was talking about the way the Senate used to work. That’s the way the Senate used to work.”