Former vice president Joe Biden is in hot water again for appearing to be flip about something in his past that his primary opponents think is much more serious — first it was about touching women, and now it’s about his work with segregationist senators in the ’70s.

There’s a lot at stake for Biden. It has allowed some of his opponents — specifically Sen. Cory Booker (D-N.J.) — to land blows against him, right before the Democratic debates, when many candidates will want to take shots at Biden to try to lower his standing in the polls. This confrontation also could endanger Biden’s central pitch to voters: that he can unify the country after President Trump. Here’s what’s going on and why Biden is so defensive about it:

On Tuesday, Biden championed how he worked with racists in the Senate: He said at a fundraiser in New York that he’s so good at bringing people together, he developed relationships with segregationists in the Senate in the 1970s, mentioning two by name. “He never called me ‘boy,' " Biden said of Sen. James O. Eastland of Mississippi. “He always called me ‘son.' ”

He’s made such comments before: But not when he was running for president, and not right before a major debate where candidates trailing him in the polls want to get themselves noticed.

Two 2020 candidates of color took major issue with what he said: And they had pretty potent responses.

“You don’t joke about calling black men ‘boys,' ” Booker said in a statement, explaining how “boy” has long been used as an epithet for black people.

Sen. Kamala D. Harris (D-Calif.) said she was “deeply” concerned by what Biden said. At the Capitol on Wednesday, she said: “If those men had their way, I wouldn’t be in the United States Senate and on this elevator right now."

Biden got angry that he was getting attacked for this: Biden has avoided talking about, much less criticizing, his 2020 opponents. But he seemed to interpret Booker’s remarks as an accusation that he’s somehow less than supportive of civil rights. (Which is not what Booker appeared to be saying.)

Biden told reporters Wednesday on his way to another 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.”

Biden also has a political incentive to fight this one to its end: He said he got into this race because of Trump’s relationship with self-professed white supremacists. Trump has struggled to disavow deadly incidents tied to them such as when he said “both sides” were at fault in the 2017 Charlottesville protests led by self-proclaimed white supremacists that left a woman dead.

"At that moment, I knew the threat to this nation was unlike any we’ve seen in our lifetime,” Biden said in his campaign launch video. “We are in a battle for the soul of this nation.”

You could argue that even bringing up segregationist senators was a gaffe on Biden’s part. Biden is trying to contrast himself with Trump on race, but he just pitched himself as friendly with someone such as Eastland, who said during World War II that black soldiers were inferior to white ones.

Biden wants voters to focus on how he disagreed with senators such as Eastland but still worked with them. It’s not clear that came across.

From Biden’s perspective, bringing up Eastland underscores his main pitch to voters: Which is that he, above anyone else, can bring the country together after Trump. If he could work with segregationists in the ‘70s, imagine what he could do today to heal this nation.

Biden has powerful Democratic backers on this: The highest-ranking African American member of Congress, House Majority Whip James E. Clyburn (D-S.C.), told reporters Wednesday he understands where Biden is coming from and that he, too, worked with people such as Strom Thurmond. Other black Democrats in Congress tried to frame Biden’s words in charitable ways.

“I think the sentiment is something that we all know in the legislative body — that you work with people you don’t agree with,” Rep. Cedric L. Richmond (D-La.), a co-chairman of Biden’s campaign, said.

The Clyburn comment in particular is helpful to Biden. If there’s one friend in Congress you want to make as a Democratic presidential candidate, it’s probably Clyburn. He rules South Carolina Democratic politics, and South Carolina is the first early voting state with a majority black Democratic primary voters. Biden and his opponents will be campaigning in South Carolina this weekend at Clyburn’s famous fish fry.

This is reminiscent of Biden’s last major dust-up involving accusations by eight women that he touched them in ways that made them feel uncomfortable. Then, as now, Biden wouldn’t apologize — he even made a joke about it. He also had powerful supporters in Congress such as House Speaker Nancy Pelosi (D-Calif.), who gave him some slack.

“I have no criticism of what he believes is his story to tell the American people, that he will work with anyone to get a good result for Americans,” Pelosi said Wednesday.

Like that moment then, it’s tough to say how the latest problem for Biden will affect his campaign. Is this the leader in the polls being unfairly attacked by his opponents? Is this a gaffe? Or a deeper wound to Biden?