Senator Kamala Harris, one of only two black presidential candidates in a field of two dozen, had a history-making moment on the debate stage on Thursday night when she challenged former vice president Joe Biden over racial issues.

But as the reaction to her debate performance poured in, so did the racist attacks on social media, where some accused the California Democrat of not being black enough, and others suggested she was not really American.

On Twitter, some commenters suggested Ms Harris was unfairly portraying herself as African American, since she is the daughter of a Jamaican father and an Indian mother.

By Saturday, Ms Harris’s campaign spokeswoman had retweeted nearly a dozen comments and articles defending her boss, and Ms Harris’s 2020 Democratic challengers forcefully condemned the attacks on social media.

“This stuff is really vile and everyone should speak out against it,” Lily Adams, Ms Harris’s spokeswoman, wrote on Twitter.

The Democrat challengers to Trump in 2020 Show all 25 1 /25 The Democrat challengers to Trump in 2020 The Democrat challengers to Trump in 2020 Bernie Sanders The Vermont senator has launched a second bid for president after losing out to Hilary Clinton in the 2016 Democratic primaries. He is running on a similar platform of democratic socialist reform Getty The Democrat challengers to Trump in 2020 Joe Biden The former vice president recently faced scrutiny for inappropriate touching of women, but was thought to deal with the criticism well and has since maintained a front runner status in national polling EPA The Democrat challengers to Trump in 2020 Elizabeth Warren The Massachusetts senator is a progressive Democrat, and a major supporter of regulating Wall Street Reuters The Democrat challengers to Trump in 2020 Amy Klobuchar Klobuchar is a Minnesota senator who earned praise for her contribution to the Brett Kavanaugh hearings Getty The Democrat challengers to Trump in 2020 Michael Bloomberg Michael Bloomberg, a late addition to the 2020 race, announced his candidacy after months of speculation in November. He has launched a massive ad-buying campaign and issued an apology for the controversial "stop and frisk" programme that adversely impacted minority communities in New York City when he was mayor Getty Images The Democrat challengers to Trump in 2020 Tulsi Gabbard The Hawaii congresswoman announced her candidacy in January, but has faced tough questions on her past comments on LGBT+ rights and her stance on Syrian President Bashar al-Assad Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Pete Buttigieg The centrist Indiana mayor and war veteran would be the first openly LGBT+ president in American history Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Deval Patrick The former Massachusetts governor launched a late 2020 candidacy and received very little reception. With just a few short months until the first voters flock to the polls, the former governor is running as a centrist and believes he can unite the party's various voting blocs AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Beto O'Rourke The former Texas congressman formally launched his bid for the presidency in March. He ran on a progressive platform, stating that the US is driven by "gross differences in opportunity and outcome" AP The Democrat challengers to Trump in 2020 DROPPED OUT: Kamala Harris The former California attorney general was introduced to the national stage during Jeff Sessions’ testimony. She has endorsed Medicare-for-all and proposed a major tax-credit for the middle class AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Bill De Blasio The New York mayor announced his bid on 16 May 2019. He emerged in 2013 as a leading voice in the left wing of his party but struggled to build a national profile and has suffered a number of political setbacks in his time as mayor AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Steve Bullock The Montana governor announced his bid on 14 May. He stated "We need to defeat Donald Trump in 2020 and defeat the corrupt system that lets campaign money drown out the people's voice, so we can finally make good on the promise of a fair shot for everyone." He also highlighted the fact that he won the governor's seat in a red [Republican] state Reuters The Democrat challengers to Trump in 2020 DROPPED OUT: Cory Booker The New Jersey Senator has focused on restoring kindness and civility in American politics throughout his campaign, though he has failed to secure the same level of support and fundraising as several other senators running for the White House in 2020 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Wayne Messam Mayor of the city of Miramar in the Miami metropolitan area, Wayne Messam said he intended to run on a progressive platform against the "broken" federal government. He favours gun regulations and was a signatory to a letter from some 400 mayors condemning President Trump's withdrawal from the Paris Climate Accord Vice News The Democrat challengers to Trump in 2020 DROPPED OUT: Kirsten Gillibrand The New York Senator formally announced her presidential bid in January, saying that “healthcare should be a right, not a privilege” Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Delaney The Maryland congressman was the first to launch his bid for presidency, making the announcement in 2017 AP The Democrat challengers to Trump in 2020 DROPPED OUT: Andrew Yang The entrepreneur announced his presidential candidacy by pledging that he would introduce a universal basic income of $1,000 a month to every American over the age of 18 Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Julian Castro The former San Antonio mayor announced his candidacy in January and said that his running has a “special meaning” for the Latino community in the US Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Marianne Williamson The author and spiritual adviser has announced her intention to run for president. She had previously run for congress as an independent in 2014 but was unsuccessful Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Eric Swalwell One of the younger candidates, Swalwell has served on multiple committees in the House of Representatives. He intended to make gun control central to his campaign but dropped out after his team said it was clear there was no path to victory Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Seth Moulton A Massachusetts congressman, Moulton is a former US soldier who is best known for trying to stop Nancy Pelosi from becoming speaker of the house. He dropped out of the race after not polling well in key states Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Jay Inslee Inslee has been governor of Washington since 2013. His bid was centred around climate change AFP/Getty The Democrat challengers to Trump in 2020 DROPPED OUT: John Hickenlooper The former governor of Colorado aimed to sell himself as an effective leader who was open to compromise, but failed to make a splash on the national stage Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tim Ryan Ohio representative Tim Ryan ran on a campaign that hinged on his working class roots, though his messaging did not appear to resonate with voters Getty The Democrat challengers to Trump in 2020 DROPPED OUT: Tom Steyer Democratic presidential hopeful billionaire and philanthropist Tom Steyer is a longtime Democratic donor AFP/Getty

“The attacks against @KamalaHarris are racist and ugly,” Senator Elizabeth Warren tweeted shortly after.

“We all have an obligation to speak out and say so. And it’s within the power and obligation of tech companies to stop these vile lies dead in their tracks.”

“@KamalaHarris doesn’t have s*** to prove,” tweeted Senator Cory Booker.

Washington Governor Jay Inslee called the attacks “racist and vile”.

Minnesota Senator Amy Klobuchar wrote: “These troll-fuelled racist attacks on Senator @KamalaHarris are unacceptable. We are better than this (Russia is not) and stand united against this type of vile behaviour.”

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,” Ms Orr wrote, with images of the tweets questioning Harris’ racial credentials.

At Thursday’s debate, Ms Harris told Mr Biden that his past stance against federally-mandating bussing 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 Ms Harris couldn’t lay claim to the afflictions of African Americans.

President Donald Trump’s son, Donald Trump Jr 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, Mr Trump Jr tweeted: “Is this true? Wow.”

“This is the same type of racist attacks his father used to attack Barack Obama,” Ms Adams told The Washington Post. “It didn’t work then and it won’t work now.”

Two of Ms Harris’s Democratic primary opponents reacted to Trump Jr’s comment.

Donald Trump Jr says his family has lived an 'incredible immigrant story'

Bernie Sanders tweeted: “Donald Trump Jr is a racist too. Shocker.”

South Bend, Indiana, Mayor Pete Buttigieg wrote: “The presidential competitive field is stronger because Kamala Harris has been powerfully voicing her Black American experience. Her first-generation story embodies the American dream. It’s long past time to end these racist, birther-style attacks.”

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 United States to immigrants.

Ms Harris is the daughter of an Indian mother and a Jamaican father.

When she’s asked how she identifies herself, Ms 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, Ms 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,” Ms 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.”