Several Democratic presidential candidates defended fellow 2020 contender Sen. Kamala Harris on Saturday against false online accusations about her race and U.S. citizenship — with many calling the attacks racist.

The big picture: A social media researcher, cited in the NYT and BuzzFeed, aggregated at least 12 tweets questioning Harris' race during the first Democratic primary debate, saying "it has all the signs of being a coordinated/artificial operation." Gary Wilmot, a self-described "birther" conspiracy theorist, made accusations about Harris' race in 2017 — sentiments shared by the Daily Stormer, a neo-Nazi website.

Donald Trump Jr. shared a tweet written by an "alt-right fringe figure" Friday that falsely claimed Harris "was not black enough to be discussing the plight of black Americans," per the New York Times. He deleted the tweet by the end of the night.

"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. And once he saw that folks were misconstruing the intent of his tweet, he quickly deleted it."

— Don Jr.'s spokesman Andy Surabian's statement to the Times

Context: On debate night, Harris, who has a Jamaican father and Indian mother, drew on her childhood experiences with racial segregation to confront former Vice President Joe Biden on his opposition to federally mandated busing for school integration in the 1970s.

What they're saying:

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

"The same forces of hatred rooted in 'birtherism' that questioned Barack Obama's American citizenship, and even his racial identity, are now being used against Senator Kamala Harris. It’s disgusting and we have to call it out when we see it. Racism has no place in America." Sen. Elizabeth Warren tweeted, " The attacks against Kamala Harris are racist and ugly. 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."

The attacks against Kamala Harris are racist and ugly. 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." Sen. Bernie Sanders tweeted, "Donald Trump Jr. is a racist too. Shocker."

"Donald Trump Jr. is a racist too. Shocker." Sen. Cory Booker tweeted, "Kamala Harris doesn’t have shit to prove."

"Kamala Harris doesn’t have shit to prove." Southbend, Indiana, Mayor Pete Buttigieg tweeted, "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."

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

"These troll-fueled racist attacks on Senator Kamala Harris are unacceptable. We are better than this (Russia is not) and stand united against this type of vile behavior." Former Rep. Beto O'Rourke tweeted, "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. Kamala Harris is an American. Period. And all of us must call out attempts to question her identity for what they are: racist."

"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. Kamala Harris is an American. Period. And all of us must call out attempts to question her identity for what they are: racist." Washington Gov. Jay Inslee tweeted, "The coordinated smear campaign on Senator Kamala Harris is racist and vile. The Trump family is peddling birtherism again and it’s incumbent on all of us to speak out against it."

"The coordinated smear campaign on Senator Kamala Harris is racist and vile. The Trump family is peddling birtherism again and it’s incumbent on all of us to speak out against it." Rep. Tim Ryan tweeted, "The attack on Kamala Harris is racist and we can't allow it to go unchecked. We have a responsibility to call out this birtherism and the continued spread of misinformation."

Go deeper: Kamala Harris emerges from debate as Twitter victor

This article has been updated with new details, including comments by Democratic candidates who've defended Harris.