By Matt Viser, Annie Linskey, Washington Post–

An impassioned argument over racism in America has boiled to the surface in an increasingly muddled Democratic presidential primary contest, triggered by an electrifying debate encounter that has reverberated across the campaign for days.

Former vice president Joe Biden, who since his April 25 entry into the presidential contest has held a consistent lead in polls, is experiencing the most unsteady period of his campaign so far in the aftermath of his heated exchange with Sen. Kamala D. Harris (D-Calif.) over his past views on school desegregation, with demands from some civil rights leaders for him to further clarify his 1970s-era opposition to mandatory busing.

The moment has taken on particular salience because it showcased Biden, 76, who has premised his candidacy in part on his ability to win back working-class white voters, under attack from one of the country’s most prominent black political figures in Harris, who addressed the issues in deeply personal terms.

Harris and a second black presidential candidate, Sen. Cory Booker (D-N.J.), have spent the past week casting Biden as an anachronism who should apologize for his busing position and his willingness to work with segregationist senators.

The rising tensions have the potential to reshape the race, with Biden at risk of losing support from crucial African American voters and Harris appearing to gain momentum. But the emerging dynamics are also sparking concern among some in the party who fear that renewing painful debates over school busing risks turning off centrist voters whom Democrats hope to win next year in their shared goal of defeating President Trump.

“There is a narrow pathway if we continue on this path of being the quote-unquote Whole Foods party,” said J.D. Scholten, the Democrat who nearly defeated Rep. Steve King (R-Iowa) in a rural district. “. . . We have to compete in places that I call Dollar General districts.”

Others, however, say that the debate is precisely what is necessary as the party looks to nominate its next standard-bearer.

“It’s not really about busing. It’s about what their commitment to racial justice looks like,” said Aimee Allison, the founder and president of She the People, a group intent on bolstering women of color in politics. “We should have a conversation that puts racial justice in the middle of a conversation in the Democratic primary.”

A survey released by CNN on Monday showed that Biden had dropped 10 percentage points among Democrats or Democratic-leaning independents since the last CNN poll in May. At the same time, the new CNN poll found that Harris had jumped nine points, and Sen. Elizabeth Warren (Mass.) eight points.

Overall, 22 percent supported Biden, while 17 percent were with Harris, 15 percent with Warren, and 14 percent with Sen. Bernie Sanders (I-Vt.). No one else in the field was above 5 percent — including South Bend, Ind., Mayor Pete Buttigieg, who also suffered a significant drop.

In an indication of how important the current debate on race is to Biden, his strongest support still comes from black voters. The survey found that 36 percent of black voters prefer him, compared with 24 percent of white ones.

The episode that kicked off days of Democratic angst was the most memorable exchange from four hours of debating over two nights, as Harris not only confronted Biden’s long-held opposition to school busing but also personalized it by saying that she herself benefited from a busing system in California.

Biden said she was misinterpreting his position, but he struggled to respond in a debate that even supporters concede went worse than expected. The next day, in an appearance in Chicago, he spoke more forcefully when reading a speech via a teleprompter in which he defended his long record on voting rights and said the civil rights struggle was what drove him to run for office nearly five decades ago.

But Biden’s speech also highlighted the complicated nature of his argument, as he sought to hold the same position he held in the 1970s and yet square it with today’s politics on race. He has been an opponent of federally mandated busing — calling it an “asinine concept,” a “liberal train wreck” and discussing a constitutional amendment to ban it. But recently, he has drawn the distinction that he never opposed having school districts start a busing system on their own.

His campaign on Monday stressed that he also supports policies to address segregation, including rezoning school districts, combating discriminatory housing policies and eliminating obstacles to new public housing.

“He also believes that in 2019, there should be first-rate, quality schools in every neighborhood in America — and by tripling Title 1 funding, his education plan includes the resources to make that happen,” campaign spokesman T.J. Ducklo said.

“I don’t rule out that he can redeem himself,” said the Rev. Al Sharpton, who in April hosted many in the Democratic field before the National Action Network. “But he’s going to have to do some clarifying.”

Sharpton said he had issues with Biden’s position against busing as well as with his response to Harris that he was supportive of local districts implementing such programs — but not supportive of the federal government intervening in busing.

“His reflexive answer was more problematic than the initial answer,” Sharpton said. “To say you agreed with the locals making the decision and not the federal government is the problem. . . . What you are now advocating is states’ rights, which is what we fought the last 50 years. Hell, that’s what the Civil War was about.”

It has been a striking turn that a little over two years after the first black president left office, his party is now consumed by a major dispute over decades-old policies aimed at desegregation — and the opposition to those policies by that man’s vice president. Civil rights leaders who have worked with and admired Biden have been surprised, both by his debate performance and his struggles to move past it.

“Someone like Biden who actually has gone through a son’s death and has gone through pain, the fact that he was pretty flat-footed was a surprise to people,” said Randi Weingarten, president of the American Federation of Teachers. “The fact that he didn’t hear her pain was a surprise to people, and it was a surprise to me.”

“I do think he knows and he can deal with the issue of pain,” Weingarten said. “But I think, like so many others, why, why not just say: ‘I’m sorry.’ ”

The debate exchange has also animated Biden’s supporters, who are angry that Harris has used what they view as an unfair attack to rise in the polls.

“This was strategically planned,” said Harold Schaitberger, president of the International Association of Firefighters, which has endorsed Biden. “This wasn’t an organic reaction. Like, ‘I’m offended and I’m organically emotionally reacting to this.’ This was a contrived, rehearsed, premeditated ambush. And I’m sorry that’s the case. I know the senator well and I have — I shouldn’t say had — I have respect for her. But I don’t think this will be looked upon as her finest moment.”

“Anybody who knows Joe Biden and his career knows his long and profound support of civil rights in every way you can measure it,” he added.

Harris’s debate thrust put Democratic candidates in a position they have not experienced for decades — having to contemplate a position on busing. Harris on Sunday said that she still supports busing and sees a modern-day use for it.

“The schools of America are as segregated, if not more segregated, today than when I was in elementary school,” she told reporters in San Francisco on Sunday. “And we need to put every effort, including busing, into play to desegregate the schools. . . . There’s no such thing as separate but equal, and so busing is one of the ways by which we create desegregation and we make it more equal.”

Harris’s campaign on Monday said that she supported federal resources for busing and cited legislation from Sen. Chris Murphy (D-Conn.) and Rep. Marcia L. Fudge (D-Ohio). That legislation would authorize $120 million for a variety of purposes, including one to expand school busing services.

Some in the party worry about a debate over busing policies — court-ordered school busing was highly unpopular among white voters in the 1970s, and black voters were divided on the practice. In today’s context, some worry that a return to advocating it might not play well among suburban voters who powered Democratic House wins in 2018. Others want to focus more broadly on racial justice — an area that Biden, the author of a tough-on-crime bill from 1994, could also struggle with.

Already, however, the debate is playing out in unpredictable ways. Gloria Major, a 66-year-old black voter from Columbia, S.C., grew up being bused to a white school in Florida.

“Racism is very real to me, but the realest thing is this country is going to hell in a handbasket,” she said. “Why are you debating something that happened 40 years ago? That came out of the clear blue sky?”

Before the debate, she said, Harris was among a quartet of candidates who intrigued her. After the debate, Harris was off the list for creating “a distraction.”

“Why would I hold him accountable for something that happened in 1975?” she said. “Let’s go pull out all the skeletons out of everybody’s closet if that’s the case.”

“I feel like you’re being distracted from the issues that matter, and I think she’s trying to boost herself in the race,” she added. “There’s other things: Why not find the issues that are right now out of hand? We’re being sold to Russia and China. What about unarmed black men being shot by police? What about body cameras?”

matt.viser@washpost.com

annie.linskey@washpost.com

Cleve R. Wootson Jr. and Chelsea Janes contributed to this report.