New Jersey U.S. Senator Cory Booker is backing up one of his rivals in the U.S. presidential race, after her comments on race prompted online attacks on her characterization of her own.

".@KamalaHarris doesn’t have s--t to prove," Booker wrote on Twitter Saturday evening, in response to a New York Times report. His tweet did not censor any profanity.

That report detailed how Donald Trump Jr. — the president's son — shared but later deleted another person's tweet criticizing Harris' discussion of her race.

"Kamala Harris is implying she is descended from American Black Slaves," Ali Alexander, whose Twitter biography says he "exposed Kamla Harris," wrote. "She’s not. She comes from Jamaican Slave Owners. That’s fine. She’s not an American Black. Period."

Harris, in one of this week's two Democratic primary debates, challenged former Vice President Joe Biden on his recent comments saying he'd had civil conversations and worked productively with segregationists.

"I was actually very — it was hurtful to hear you talk about the reputations of two United States senators who built their reputations and career on the segregation of race in this country," she said in the debate. "And it was not only that, but you also worked with them to oppose busing. And, you know, 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."

Biden countered that in he had been against "busing ordered by the Department of Education," not busing overall, though several letters of his at the time show broader ppposition to busing, CNN has since reported.

Trump Jr., in retweeting Alexander, initially wrote  “Is this true? Wow," but then later deleted his tweet.

“Don’s tweet was simply him asking if it was true that Kamala Harris was half-Indian because it’s not something he had ever heard before,” spokesman Andy Surabian told the New York Times. “And once he saw that folks were misconstruing the intent of his tweet, he quickly deleted it.”

Harris is the Oakland, California-born daughter of a Jamaican father and Indian mother. She described herself at the debate, as she has in other public statements, as black — saying a neighbor was told by her parents she couldn't play with Harris and their sister because they were black. She did not describe herself as descended from American black slaves.

Booker — who took part in the first of the two nights of debate, and who hasn't yet appeared on a debate stage with Harris or Biden — also this week criticized Biden for saying “We’ve got to recognize that kid wearing a hoodie may very well be the next poet laureate and not a gangbanger.” Biden made those remarks at a Rainbow PUSH luncheon, advocating for criminal justice form, a cause Booker has also championed, CNN and others reported.

"This isn't about a hoodie," Booker tweeted. "It's about a culture that sees a problem with a kid wearing a hoodie in the first place. Our nominee needs to have the language to talk about race in a far more constructive way."

Booker, in an appearance on CNN, also criticized Biden's support of a 1994 crime bill many have since criticized as ushering in an era of mass incarceration, and said Biden's previous comments about segregationists revealed a lack of understanding of the racial dynamics in play.

"This is a lesson on some of the issues that a nominee shouldn't at this point have to learn," he said.

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