Former vice president Joe Biden defends his record on racial issues in Thursday night's debate with Sen. Kamala D. Harris (D-Calif.), as Sen. Bernie Sanders (I-Vt.) is caught in between. (Mike Segar/Reuters)

An exchange between Sen. Kamala D. Harris (D.-Calif.) and former vice president Joe Biden during the second night of the first Democratic primary debate encapsulated months of building tensions within the party over race and generational divisions, turning a spotlight onto Biden’s history on busing to integrate schools.

It was the first big face-to-face showdown of the primary over the role that the intersection of race and policy will play in the 2020 election.

Harris confronted Biden directly about his past stance on school busing. Biden was quick to defend himself. Here’s the exchange:

Harris: Vice President Biden, do you agree today that you were wrong to oppose busing in America then?

Biden: I did not oppose busing in America; what I opposed is busing ordered by the Department of Education.

Harris: Well, there was a failure of states to integrate public schools in America. I was part of the second class to integrate Berkeley, California, public schools almost two decades after Brown vs. Board of Education.

Biden: Because your city council made that decision. It was a local decision.

Harris: So that’s where the federal government must step in. That’s why we have the Voting Rights Act and the Civil Rights Act. That’s why we need to pass the Equality Act. That’s why we need to pass the ERA, because there are moments in history where states fail to preserve the civil rights of all people.

Harris pointed out that she was bused as child in the in Berkeley, Calif., school system.

The Washington Post’s Matt Viser wrote in March that when Biden was a freshman senator in the mid-1970s, he took a lead role in the fight about school busing, repeatedly speaking out against sending white children to predominantly black schools and black children to predominantly white schools.

Harris’s decision to directly confront Biden over busing, in personal terms, was notable and likely to garner a slew of headlines.

Given the how much you’re likely to hear about the exchange, it’s worth reading Viser’s whole piece on Biden, but here are a few key passages that illuminate the vulnerability Harris saw:

Biden took a lead role in the fight [over school busing in Delaware], speaking out repeatedly and forcefully against sending white children to majority-black schools and black children to majority-white schools. He played down the persistence of overt racism and suggested that the government should have a limited role in integration.

“I do not buy the concept, popular in the ’60s, which said, ‘We have suppressed the black man for 300 years and the white man is now far ahead in the race for everything our society offers. In order to even the score, we must now give the black man a head start, or even hold the white man back, to even the race,’ ” Biden told a Delaware-based weekly newspaper in 1975. “I don’t buy that.”

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Although civil rights leaders may object to Biden’s past statements about busing, his decision to stand by his views on the issue illustrate what some of his supporters think would be his advantage in the 2020 field: his ability to appeal beyond the Democratic base to some working-class white voters who voted for Donald Trump in 2016.