The most personal attack of Thursday's Democratic debate was aimed not at President Donald Trump, but at former Vice President Joe Biden.

California Sen. Kamala Harris called out Biden on the issue of race, saying the way he recently described his past working relationship with two segregationist lawmakers was "hurtful."

Biden, at a campaign event earlier this month, had cited his ability to get things done even with segregationist senators as examples of the type of "civility" in Congress that has since disappeared. Numerous Democratic presidential candidates took Biden to task for the comments, though som, including civil rights icon John Lewis, defended the 2020 frontrunner's remarks.

Harris also accused Biden of opposing busing, which Biden disputed.

"There was a little girl in California who was part of the second class to integrate her public schools, and she was bused to school every day, and that little girl was me," Harris said.

"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."

Harris' official Twitter account quickly followed up, posting a photo of the candidate as a child.

Harris TWEET

Biden called Harris' attack "a mischaracterization of my position across the board," and launched into a fiery defense of his record on race. "I did not praise racists."

"If we want to have this litigated on who supports civil rights, I'm happy to do that," Biden continued.

"I was a public defender. I was not a prosecutor," he added — a clear shot at Harris, who had been San Francisco's district attorney and California's attorney general.

Biden's campaign did not immediately respond to CNBC's request for comment about the exchange. But Cristobal Alex, a senior advisor to Biden's campaign, tweeted that it was "outrageous" to attack President Barack Obama's veep "because he worked to convince" Republicans to pass civil rights legislation.

Cristobal Alex TWEET

Harris followed up after Biden defended himself. "Do you agree today that you were wrong to oppose busing in America?"

"I did not oppose busing in America," Biden responded as his voice grew noticeably louder. "What I opposed is busing ordered by the Department of Education."

"I have supported the [Equal Rights Amendment] from the beginning," Biden said. "I'm the guy that extended the Voting Rights Act for 25 years, we got to the place where we got 98 out of 98 votes in the United States senate doing it. I've also argued very strongly that we in fact deal with the notion of denying people access to the ballot box."

Biden then stopped himself. "Anyway, my time's up," he said, referring to time constraints under the debate rules.