Days after Donald Trump Jr. shared, and then deleted, a tweet questioning Senator Kamala Harris’s race, several of her rival Democratic presidential candidates lent her their support on Saturday.

Mr. Trump, President Trump’s eldest son, on Thursday had shared a tweet from Ali Alexander, a right-wing personality, that falsely claimed Ms. Harris’s racial identity did not qualify her to speak about the anguish that black Americans face.

“Kamala Harris is implying she is descended from American Black Slaves,” Mr. Alexander wrote during the second night of the Democratic debates. “She’s not. She comes from Jamaican Slave Owners. That’s fine. She’s not an American Black. Period.”

Mr. Trump shared the message, asking his more than three million followers, “Is this true? Wow.”

By Saturday, several Democratic hopefuls had begun to publicly support Ms. Harris, of California, who is the biracial child of a Jamaican father and an Indian mother.