(AP photo)

WASHINGTON (AP) — Kamala Harris spoke slowly but bluntly as she stared at Joe Biden, then began treating him as a hostile witness.

The former prosecutor turned California senator started by saying she didn't think the former vice president "was a racist." But she criticized him for recently "defending segregationists" in the Senate and for once opposing mandatory busing of students to desegregated public schools.

Harris described a young girl in the 1970s who boarded such buses before dramatically offering, "That little girl was me."

The moment was as powerful as it was unexpected, a searing line of attack against Biden, who served as vice president to the first black president. Biden entered back-to-back nights of Democratic presidential debates in Miami as the leading Democratic candidate. Harris showed promise but had not made much of a mark lately.

That changed Thursday.

That Harris and other Democratic presidential hopefuls would come out swinging against Biden was no surprise, and her verbal strike was hardly spontaneous. Moments after the exchange, her campaign tweeted a picture of a school-age Harris with pigtails, over the caption: "There was a little girl in California who was bussed to school. That little girl was me."

In deeply personal tones, Harris hammered Biden for policy choices that she suggested betrayed the spirit of the civil rights movement, if not directly opposing all it stood for. Then she really hit her stride, exhibiting the controlled force of a practiced cross-examiner.