WASHINGTON — California Sen. Kamala Harris' rivals for the Democratic presidential nomination defended her after criticisms based on her race circulating on social media, which were amplified when Donald Trump Jr. shared one such post with his more 3.6 million followers on Thursday.

Trump Jr. retweeted a post from Ali Alexander – a conservative commentator whose Twitter profile claims to have "exposed Kamala Harris" – that said "Kamala Harris is *not* an American Black. She is half Indian and half Jamaican. I'm so sick of people robbing American Blacks (like myself) of our history. It's disgusting."

"Is this true? Wow," President Donald Trump's oldest son wrote, before deleting the post that same night, according to The New York Times.

Tucker Carlson comments on Cory Booker's race:Fox News host calls Booker one of 'whitest candidates' among 2020 Democrats

"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," Andy Surabian, a spokesman for Trump Jr. told the Times. "And once he saw that folks were misconstruing the intent of his tweet, he quickly deleted it."

The 2020 candidates:Who is running for president? An interactive guide

Harris, 54, was born in Oakland, Calif. Her mother, Shyamala Gopalan came to America from India where she met Donald Harris, an immigrant from Jamaica. Both civil rights activists, they met while pursuing graduate studies at the University of California, Berkeley, Harris said in her memoir, "The Truths We Hold: An American Journey."

Many took note of Trump Jr.'s retweet, including those competing against Harris for the Democratic nomination. Harris' husband, Douglas Emhoff thanked "the 2020 candidates and everyone else for calling out this crap for what it is."

"Donald Trump Jr. is a racist too. Shocker," Sen. Bernie Sanders, I-Vt., said on Twitter Saturday, along with a link to the Times story about Trump Jr.'s retweet.

Former Housing and Urban Development Secretary Julián Castro called the attacks on Harris "disgusting" during an interview with CNN's "State of the Union" on Sunday and called Trump Jr. a "coward" for his tweet.

"He knows what he's doing," Castro said. "He's giving voice to these racist utterances about Senator Harris. We need to dispel them immediately and condemn them, and then not give them any more life, because they're disgusting."

"The attacks against @KamalaHarris are racist and ugly," said Sen. Elizabeth Warren, D-Mass, in a tweet. "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."

"This is racism. It was wrong before, and it’s wrong now. We won’t allow it again," Sen. Kirsten Gillibrand, D-N.Y., said on Twitter.

"There's a long history of black Americans being told they don't belong – and millions are kept down and shut out to this day. @KamalaHarris is an American. Period. And all of us must call out attempts to question her identity for what they are: racist," said former Texas Rep. Beto O'Rourke.

Montana Gov. Steve Bullock said the attacks on Harris "represent the worst of us."

Sen. Cory Booker, D-N.J., put it more simply, tweeting that Harris "doesn’t have s--- to prove."

Former Vice President Joe Biden compared the budding social media campaign about Harris' background to the "birtherism" conspiracy theory that questioned former President Barack Obama's citizenship and eligibility to serve as president.

Donald Trump, then a reality TV star, was a leading voice of birtherism during the Obama era.

"The same forces of hatred rooted in 'birtherism' that questioned @BarackObama's American citizenship, and even his racial identity, are now being used against Senator @KamalaHarris. It’s disgusting and we have to call it out when we see it. Racism has no place in America," Biden said in a tweet.

Biden and Harris had a feisty exchange on the issue of race and racism during the first Democratic debate of the 2020 campaign season.

Harris campaign spokeswoman Lily Adams weighed in on Donald Trump Jr.'s retweet.

"This is the same type of racist attacks his father used to attack Barack Obama. It didn’t work then and it won’t work now," Adams said in a statement.

South Bend, Indiana, Mayor Pete Buttigieg also compared the attacks to birtherism.

"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," Buttigieg said in a tweet.

Similarly, Washington Gov. Jay Inslee called it a "coordinated smear campaign" and said the "Trump family is peddling birtherism again and it’s incumbent on all of us to speak out against it."

"It is more than 'birtherism again' to question the race of @KamalaHarris. It’s racism," said Rep. Eric Swalwell, D-Calif.

Alexander said in a statement that it was Harris' claim to African-American identity during the debate that he was denouncing, and denied that he was trying to make any kind of "birtherism" argument, acknowledging that Harris was born in Oakland, California to parents "who were naturalized citizens."

“Me pointing out that Kamala Harris has a mother from India and a father from Jamaica went viral last night because many people assume she descends from Black American Slaves,” he said. "She does not. I corrected Kamala Harris last night because she stole debate time under the premise that she is an African-American when she is in fact a biracial Indian-Jamaican who is a first generation American."

Some on social media said the viral spread of Alexander's post appeared to be aided by artificial accounts, commonly referred to as bots. Such accounts were central to the social media element of Russia's "sweeping and systematic" effort to interfere in the 2016 election, according to the special counsel Robert's Mueller's report.

One post by freelance journalist Caroline Orr was cited by several news publications.

"A lot of suspect accounts are pushing the 'Kamala Harris is not Black' narrative tonight. It’s everywhere and it has all the signs of being a coordinated/artificial operation," Orr posted after the debate.

Sen. Amy Klobuchar, D-Minn., indicated she believed bots played a role in disseminating Alexander's tweets.

"These troll-fueled racist attacks on Senator @KamalaHarris are unacceptable. We are better than this (Russia is not) and stand united against this type of vile behavior," Klobuchar tweeted.

Rep. Seth Mouton, D-Mass., said "these attacks are being promoted by the same kind of bot activity the Russians and others used to hijack our democracy in 2016. Don't fall for it."

But a Twitter spokesperson told The Hill, "Reports of malicious automation and 'bots' are false."

"We've see no coordinated activity around the debates, or this particular content. It is organic users sharing these views, much of which is not in violation of our policies."

Harris is not the only candidate whose identity as an African-American has been questioned by conservatives. After Wednesday night's debate, Fox News host Tucker Carlson referred to Booker, who is black, as one of "the two whitest candidates on stage" in a screed against Booker and O'Rouke's use of Spanish.