Sen. Cory Booker speaks at the National Urban League Conference in Indianapolis on Thursday, as Marc Morial, president of the group, listens. (Darron Cummings/AP)

As the fight to become the nominee of an increasingly diverse Democratic Party roars toward a second debate, Joe Biden has repeatedly found himself in bitter conflicts with the two most prominent black presidential candidates, Sens. Cory Booker (N.J.) and Kamala D. Harris (Calif.).

Their progressively ­high-profile battling carries risks for all parties involved, with Biden growing far more defiant in his responses, and Booker and Harris making more explicit overtures to black voters as a way to loosen the former vice president’s grip on a bloc that has kept him atop the polls.

The dispute marks a critical chapter in the fight for the soul of a party that was most recently guided by the nation’s first black president — and whose vice president is trying to ride some of that lasting goodwill — but the debate now is becoming more raw, less predictable, and with potentially perilous fault lines for both sides.

It will play out more prominently next week on the debate stage, with Biden standing between Harris and Booker for two hours with the potential of condensing weeks of criticism.

Booker in particular has escalated his criticism of Biden, suggesting that his record on race and criminal justice should be disqualifying for a Democratic nominee in 2020. Citing Biden’s role crafting a 1994 crime bill, Booker has called him “an architect of mass incarceration” and said that it is time for a new generation to dismantle what Biden created.

“It is easy to call Donald Trump a racist now. You get no badge of courage for that,” Booker said Thursday before a National Urban League meeting in Indianapolis. “The question is, what were you doing to address structural inequality and institutional racism throughout your life? Don’t just tell us what you’re going to do. Tell us what you’ve already done. Don’t just tell us you’re going to be a champion for our communities when you become president, if you haven’t been a champion already.”

Joe Biden speaks at the National Urban League Conference. “I’ve been in this fight for a long, long time,” he said. (Darron Cummings/AP)

The spat between Biden and Booker follows one last month in which Booker called on Biden to apologize for touting his ability to work with segregationist senators. Biden responded by calling on Booker to apologize before, two weeks later, ultimately offering a conditional apology himself. Harris, meantime, took on Biden in the first debate in June over his opposition to federal busing to achieve integration.

The simmering debate — playing out amid the most diverse presidential field in history — contrasts with Barack Obama’s campaigns, which largely downplayed the history-busting nature of his candidacy. A University of Pennsylvania study found that Obama spoke less about race than any other Democrat in the first two years of his presidency since John F. Kennedy in the 1960s.

This year, the presidential contest has brimmed with issues touching on race, including economic plans tailored to black Americans, proposals meant to improve black maternal health and potential reparations for descendants of slaves.

“Racism was like the third rail of politics. It was something Democrats did not touch,” said Cornell Belcher, a Democratic strategist and former Obama pollster. “A decade ago, you didn’t hear any Democrat talking about systemic racism. You didn’t hear any Democrat using terms like ‘institutional racism.’ You do today. Almost every Democrat running for office is talking about the issue of systemic racism.”

Booker has repeatedly sought to distinguish himself by pointing out that he lives in inner-city Newark, an implicit reminder that he understands the problems of many black voters because he lives them every day.

Harris has bristled at questions about her identity, saying she doesn’t want to be condensed into one small box. But her campaign has begun to talk about her “black agenda,” as Harris has been asserting herself as the candidate most able to speak candidly about issues of race. She has cited her background as the daughter of civil rights activists and her career in the criminal justice system as evidence.

“She brings a lot of the same kind of essence that Obama brought, but she is a lot more authentic than he was,” said Rep. Bobby L. Rush (Ill.), a longtime civil rights activist who said he endorsed Harris in part because of the way she talks about race. Obama challenged Rush in a 2000 congressional primary, and the incumbent won in part by questioning Obama’s roots in Chicago’s black neighborhoods. Rush later endorsed Obama’s presidential campaigns.

“Some people either coddle or ignore,” Rush said. “Well, Kamala don’t coddle, and she don’t ignore. It’s a part of who she is. So she can be tough, but yet she can be very, very compassionate because she’s talking about her own experience. She understands the black person’s experience.”

During the debate last month, Harris reminded moderators that she was the only black person on the stage (Booker had participated in the previous night’s debate) and pointedly said she wanted to talk about race. She then told a personal story about being bused to school as a young girl — and directly confronted Biden for his 1970s-era opposition to mandated busing.

The impact of that moment showed other candidates the benefits of attacking Biden and caused Biden to rethink his approach to the race.

“I’m not going to be as polite this time,” Biden said at a fundraiser in Detroit on Wednesday night. “If they want to argue about the past, I can do that. I’ve got a past I’m proud of. They’ve got a past that’s not quite so good.”

Biden has sharply criticized Booker, pointing to his support as Newark mayor for a “zero tolerance policy” for minor infractions, as well as the police department’s reliance on “stop and frisk,” a practice that has long been condemned by Democrats and civil rights activists.

He has also said he was taken aback by Harris’s criticisms, and he has pointed to his past support for her U.S. Senate race in 2016.

“I thought we were friends,” Biden said during an interview that aired Thursday on “The Tom Joyner Morning Show.” “And I hope we still will be.”

For Biden, the danger of the feuds is alienating black voters; for Harris and Booker, it is becoming identified solely as representatives of black Americans.

Voters so far have been pragmatic in their choices, which has given Biden a lead in most polls because voters see him as the most likely to defeat Trump. In a recent Quinnipiac poll, he was the only one in the field ahead of Trump in Ohio.

A new survey released on Thursday showed Biden continued to be buoyed by solid support among black Democratic voters in the first state to vote in the primaries that is dominated by black voters, South Carolina. Biden was the pick for 39 percent of likely primary voters, compared with 12 percent for Harris, 10 percent for Sen. Bernie Sanders (Vt.), and 9 percent for Sen. Elizabeth Warren (Mass.).

Biden had 51 percent support among black Democrats — a group that makes up more than 6 in 10 likely primary voters. He also had the most favorable rating — 82 percent favorable, 4 percent unfavorable — with black voters.

One reason appeared to be that black Democrats were far more likely than whites to identify as moderate or conservative, thus closer to Biden on the ideological span. The survey found that 76 percent of black voters saw themselves as centrists, compared with 43 percent among white voters.

The challenge can be seen through voters like Marvin L. Price, the president of the Central Carolina Young Professionals. His top two choices right now are Harris and Biden. “I think it’s a two-edged sword,” he said of the challenges to Biden’s record. “You have to own what you’ve done in the past, but at the same time you have to be able to right your wrongs moving forward. I do feel like right now both sides had a point.”

At the National Urban League meeting, Biden earned the most enthusiastic reception of the five presidential candidates who took the stage and played himself up as a familiar, friendly face for African American voters.

“This is all familiar commitment to me, but it’s real,” Biden said, after outlining his history with the group, citing a lifetime achievement award it gave him recently and dropping the names of a few well-known figures along the way. “I’ve been in this fight for a long, long time,” Biden said.

Both Biden and Booker have been advocating decorum among the candidates, which has made their recent outbursts against each other more surprising. Booker was asked Thursday if he still thinks everyone should try to get along.

“Absolutely,” he said. “We have to talk about how we fight. We can’t mistake strength with cruelty. We can’t mistake tough with meanness . . . If you want a fight-fire-with-fire candidate, don’t choose me.”