Much of 2020 Democratic presidential field closed ranks around fellow candidate Kamala D. Harris this week, denouncing a now-deleted tweet from Donald Trump Jr. as a racist attack on the California senator.

“The attacks against Kamala Harris are racist and ugly,” Sen. Elizabeth Warren tweeted. “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. Cory A. Booker, who is also black, tweeted that Ms. Harris “doesn’t have s—t to prove.”

Earlier this week, President Trump’s eldest son retweeted a post from Ali Alexander, a right-wing personality, that questioned Ms. Harris‘ identity as a black woman.

“She comes from Jamaican Slave Owners,” the tweet read. “That’s fine. She’s not an American Black. Period.”

Researchers concluded that similar tweets questioning Ms. Harris‘ race were being spread on social media by bots.

The post originally went up during the debate and Mr. Trump later retweeted it. A spokesman for Mr. Trump told The New York Times it was a misunderstanding.

Ms. Harris‘ parents were Jamaican and Indian.

Several other 2020 candidates have since weighed in, likening the controversy to the birther conspiracy aimed at former President Barack Obama, including former Vice President Joseph R. Biden, Rep. Tim Ryan and Gov. Jay Inslee.

“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,” Mayor Pete Buttigeig tweeted. “It’s long past time to end these racist, birther-style attacks.”

Before seeking the presidency himself, Donald Trump repeatedly questioned Mr. Obama’s American citizenship and demanded the former president reveal his birth certificate.

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