Sen. Kamala Harris' running mates for the 2020 presidential campaign are coming out to defend the California lawmaker in the wake of a racist conspiracy around her ethnic background.

On Thursday, Ali Alexander, a fringe alt-right activist, tweeted that Harris is not a black American. Harris was born in Oakland, California, to a Indian mother and a black Jamaican father, both of whom immigrated to the United States.

Alexander claimed that Harris "comes from Jamaican Slave Owners," and therefore can't speak out on race. At the time of the tweet, Harris was debating former Vice President Joe Biden on his views and track record on race, a portion of the debate that led to her best-ever day of fundraising.

Donald Trump Jr., the president's eldest son, shared that tweet to his 3.65 million followers. He later deleted it, but it nevertheless helped spur the racist conspiracy.

Some, including the Harris campaign, are comparing the idea that Harris is not actually black to the years-long "birtherism" conspiracy aimed at discrediting former President Barack Obama.

"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," Harris campaign spokeswoman Lily Adams said.

Read more: Donald Trump Jr. shared a racist tweet about Kamala Harris and then deleted it after it went viral, fanning the flames of a new 'birther' conspiracy

And Harris's running mates are coming out against the tweet — particulary Sen. Cory Booker, who is the only other black American running in the 2020 campaign.

Former Secretary of Housing and Urban Development Julián Castro, another presidential candidate, also slammed the growing conspiracy, calling it "disgusting and racist."

And Sen. Bernie Sanders took aim at Trump Jr. himself, blasting the tweet.

Andy Surabian, Trump Jr.'s spokesperson, told the New York Times that the president's son shared the tweet out of simple curiosity.

"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," Surabian said.