"@KamalaHarris doesn't have s--- to prove," tweeted Senator Cory Booker.
Washington Governor Jay Inslee called the attacks "racist and vile."
Caroline Orr, a behavioural scientist who studies the spread of disinformation online, noted on Twitter a surge of related anti-Harris tweets that posted within minutes of each other during the debate.
"Efforts to attack Kamala Harris' race have been around for a while, but a huge volume of tweets pushing this manufactured narrative appeared tonight right after Kamala pointed out that she was the only Black woman onstage," Orr wrote, with images of the tweets questioning Harris's racial credentials.
At the debate, Harris told Biden that his past stance against federally-mandating busing black students to white schools was personally hurtful because she had benefited from that educational opportunity as a little girl. Afterward, some people on social media falsely claimed that Harris couldn't lay claim to the afflictions of African Americans.
US President Donald Trump's son, Donald Trump jnr retweeted, and then deleted, an alt-right commentator named Ali Alexander, stating, "Kamala Harris is implying she is descended from American Black Slaves. She's not. She comes from Jamaican Slave Owners. That's fine. She's not an American Black. Period."
In sharing that message to his millions of followers, Trump jnr tweeted: "Is this true? Wow."
"This is the same type of racist attacks his father used to attack Barack Obama," Adams said in email to the Washington Post. "It didn't work then and it won't work now."
Other fringe social media accounts echoed the birtherism conspiracies fuelled by Donald Trump before he was in politics regarding President Barack Obama's citizenship. Now they are questioning whether Harris was eligible to run for president, calling her an "anchor baby" because she was born in the US to immigrants.
Harris is the daughter of an Indian mother and a Jamaican father. When she's asked how she identifies herself, Harris will often say, as "an American." But recently, she's leaned in more to what it meant to grow up black in the wake of the civil rights movement - she was born in October 1964, just months after the Civil Rights Act had been signed into law.
At a recent campaign event sponsored by Planned Parenthood, Harris spoke about growing up and realising that people treated her mother with less respect because of the colour of her skin.
"I remember people looking down at my mother, assuming she was somebody's housekeeper and treating her like she was a substandard person," Harris said. "She was a housekeeper, she kept our house, and happened to be a breast cancer researcher. There was an assumption that this woman had no power and should be given no power."
"I realise now that I made a decision at a young age," she said, "that I'm not going to let anybody do that to anybody else."
