WASHINGTON — Joe Biden and Sen. Cory Booker are heading for a collision at the second Democratic debate next week, and both are eager for it.

The two campaigns have gone back and forth for days arguing over who has done more to help — or hurt — African American communities. Their escalating spat, amplified by competing speeches, verbal jabs, and aides’ Twitter blasts, pits the Democratic front-runner against a New Jersey senator trying to get traction as both campaigns try to appeal to black voters, a critical part of their party’s base.

Here’s a look at the origins of their feud, why each man has embraced it, and what’s at stake when they come face to face Wednesday night on the Democratic debate stage in Detroit.

Booker needs a breakthrough

The senator has struggled to gain steam and is at risk of remaining stuck in the second tier as top contenders soak up attention, public support, and campaign money.

His campaign thought Booker had built a springboard leading into the first debate, when he forcefully criticized Biden’s paean to cooperation with segregationist senators.

But because of a random draw, Booker wasn’t on stage with Biden, who debated the next night. That’s when Sen. Kamala Harris of California stole the show (and the topic) by directly taking on the former vice president in a moment that dominated the public conversation, left Biden wounded, and again pushed Booker to the background.

Next week, Booker will be on stage with Biden and Harris, giving him a chance to confront both rivals as he tries to show he belongs in the top tier of candidates.

» READ MORE: How to watch and stream the second Democratic debate

Biden needs to show his fight

Biden remains the front-runner by nearly every measure, but has faced questions about his sharpness since that clash with Harris. His answers to the not-unexpected attack were feeble.

Now, his campaign seems determined to show that the former vice president still has plenty of fight. After weeks of largely ignoring the rest of the Democratic field, Biden has directly jabbed at Booker and Harris in speeches, and his campaign aides have thrown Twitter elbows, emphasizing Booker’s poor polling and his past support for Biden.

This time, they seem to be signaling, Biden will be ready to take on his younger competitors.

Both are relying on black voters

Biden, Booker, and Harris are all counting on black voters as a significant part of their coalitions in the Democratic primary.

That’s part of why Biden’s initial comments about the segregationists he worked with in the 1970s were so potentially explosive. (He has since apologized for using that example, and has said he was trying only to make a point about finding ways to get things done, even with people he strongly disagrees with.)

Harris, whose parents are Jamaican and Indian, has attacked Biden over his role in opposing mandated busing to integrate public schools, noting that she was bused to school growing up in California.

Booker, meanwhile, reignited his criticism of Biden this week after the former vice president rolled out his criminal justice reform plan. He did so in unusually sharp words for a senator who typically preaches cooperation and love, pointing to Biden’s role in a now-controversial 1994 crime bill.

“The proud architect of a failed system is not the right person to fix it,” Booker said in a statement. “The 1994 crime bill accelerated mass incarceration and inflicted immeasurable harm on black, brown, and low-income communities."

Booker has made criminal justice his signature issue in the Senate, and he had a major hand in pushing through a bipartisan reform bill signed by the president. He frequently emphasizes that he is the only candidate who lives in a low-income inner city, Newark, and his signature speech centers on how his family overcame discrimination, with the help of the civil rights movement, to move into an affluent, all-white New Jersey suburb.

>> READ MORE: Criminal justice bill a major marker for Cory Booker as 2020 announcement looms

On Thursday he took direct aim at Biden’s standing with black voters.

“Most of the time when somebody is asking about ‘electability,’ they’re not asking about the African American voters who make up the most reliable constituency of the Democratic Party,” Booker said in a speech to the National Urban League. “And that’s a problem. Because the truth is, we need to understand that we cannot beat Donald Trump unless we have a large, vibrant turnout in the black community.”

Biden’s camp shot back with a tweet highlighting a poll showing Biden winning support from 41% of black voters in early voting states, against 4% for Booker.

Biden has also taken aim at the zero tolerance policies and stop-and-frisk practices of the Newark police when Booker was mayor there. Stop-and-frisk is now widely condemned by civil rights groups.

“If they want to argue about the past, I can do that,” Biden said at a fund-raiser in Detroit this week. “I got a past I’m proud of. They got a past that’s not quite so good.”

Big stakes in South Carolina

Booker and Harris have both signaled that strong early showings in South Carolina, the fourth state on the Democratic campaign calendar next year, are a key to their strategies.

>> READ MORE: Cory Booker tackles race, racism as he courts black vote in South Carolina

But Biden is dominating there so far, with the help of his standing among black voters, many of whom fondly recall his days alongside former President Barack Obama.

Biden has 39% support overall in South Carolina and 51% among black voters, both best in the field, according to a poll released Thursday from Monmouth University. Booker was a distant sixth overall with 2% support.

Harris was in second place with 12 percent overall support among Democrats in the state, including 12 percent of black Democratic voters.

The three are slated to stand next to each other at center stage in Wednesday night’s debate. It’s not hard to guess what they’ll be talking about.