Sen. Kamala Harris was the first Democratic candidate to take a direct shot at former Vice President Joe Biden on Thursday’s debate, in a tense and fiery exchange over his past opposition to federally funded integrated busing as a senator in the 1970s.

The exchange started with Harris taking the frontrunner to task for his recent comments about his past good working relationships with segregationist senators. Things got even more heated when Harris proceeded to bring up another thorny racial issue from Biden’s past: In 1975, he sponsored a bill making it so federal funds couldn’t be used by schools trying to desegregate by busing minority children to predominantly white schools and vice versa.

Looking Biden directly in the eye, Harris told the former vice president this was personal and “hurtful” to her — she was part of the second class to integrate her public school in Berkeley, California.

“You also worked with [those segregationist senators] to oppose busing. And there was a little girl in California who was part of the second class to integrate her public schools and she was bussed to school every day. And that little girl was me,” Harris said, staring Biden down. “So I will tell you that on this subject, it cannot be an intellectual debate among Democrats. We have to take it seriously. We have to act swiftly.”

Her campaign soon tweeted out a photo of Harris as a young child, getting ready to go to school.

There was a little girl in California who was bussed to school. That little girl was me. #DemDebate pic.twitter.com/XKm2xP1MDH — Kamala Harris (@KamalaHarris) June 28, 2019

Biden quickly went on defense.

“I did not oppose busing in America,” Biden responded. “What I opposed is busing ordered by the Department of Education. That’s what I opposed.”

Harris pressed on, arguing the federal government had to remedy the situation because of a “failure of states to integrate public schools in America.”

Biden’s campaign later pushed back against Harris’s characterization of how his votes impacted her personally, arguing none of Biden’s votes in the Senate would have had a negative impact on the school busing program in Berkeley, where Harris was a student (the city’s integrated busing program for elementary schools was adopted in 1968; Biden was elected to the Senate in 1972).

“Sen. Harris either didn’t know or misrepresented the facts,” a Biden campaign spokesperson said in a statement to Vox. “When Biden said it wasn’t true that he supported anything that would have stopped the busing program that impacted her, he was correct. None of [Biden’s] votes would have negatively impacted the Berkeley School Busing Program.”

Harris has been open about how personal the issue of desegregated busing is to her. She brought it up during the Senate confirmation hearing of controversial Supreme Court Justice Brett Kavanaugh last year, saying the Supreme Court decision of Brown v. Board of Education opened every door for her.

“If that Court had not issued that unanimous opinion led by Chief Justice Earl Warren in that case argued by Thurgood Marshall, I likely would not have become a lawyer, or a prosecutor, or been elected district attorney or the attorney general of California,” Harris said during the hearing.

There’s a clear strategy around Harris focusing so much on the issue of race. For one, she was the only black candidate on stage, and can speak to these issues from personal experience.

But two — and perhaps more importantly — Harris is in direct competition with Biden for black voters in 2020. Biden is polling well with black voters since getting in the race, particularly older black voters who don’t want to risk electing President Donald Trump for a second term.

Even so, Biden has a problematic history on racial issues, particularly for a party that’s moved left since he entered national politics half a century ago. This was evidenced Thursday both by his past stance on desegregated school busing and his comments about the collegiality of working with segregationists in the Senate. Harris landed that point with a biting and direct challenge, one that will likely make an impression with black voters watching.

Harris and Cory Booker, another black candidate in 2020, have called Biden out on the stump in recent weeks. But Harris had an opportunity to confront Biden directly on Thursday night — and she took it.

Update: This piece was updated to include a statement from the Biden campaign.