Washington | Republican presidential nominee Donald Trump has launched a racially insensitive attack on Kamala Harris by questioning whether she is "Indian or Black", drawing a sharp reaction from his Democratic rival who termed his remarks "the same old show" of "divisiveness" and "disrespect".

Trump's controversial remarks came as the race for the November 5 presidential election gained momentum with opinion polls showing that Vice President Harris, abruptly thrust into the role of presidential candidate fewer than 10 days ago, has narrowed the gap with her Republican rival.

Trump, 78, falsely claimed Harris had only emphasised her Asian-American heritage until recently when, he said, "she became a black person" for political gain.

“I’ve known her a long time, indirectly, not directly very much, and she was always of Indian heritage, and she was only promoting Indian heritage," Trump said at the National Association of Black Journalists convention in Chicago on Wednesday.

"I didn’t know she was Black until a number of years ago when she happened to turn Black, and now she wants to be known as Black," he said during a tense appearance at the conference, where the audience at times gasped and scoffed at his remarks.

"So I don't know - Is she Indian? Or is she black?" Trump asked.

Harris’ mother, Shyamala Gopalan, was Indian and her father, Donald Jasper Harris, is Jamaican; both immigrated to the US.

As an undergraduate, Harris attended Howard University, a historically black school in Washington, and belongs to Alpha Kappa Alpha, the country's oldest Black sorority. She was also the president of the Black Law Students Association while studying at the University of California's law school in San Francisco, and a member of the Congressional Black Caucus during her time in the Senate.

When one of the journalists who was interviewing Trump on stage tried to tell him that Harris had always identified as Black and had attended a historically Black university, Trump continued: “I respect either one, but she obviously doesn’t. Because she was Indian all the way, and then all of a sudden, she made a turn, and she went - she became a Black person. And I think somebody should look into that too.”

Republican vice presidential nominee JD Vance also backed Trump's attack on Harris, saying the former president only “pointed out the fundamental chameleon-like nature of Kamala Harris” in his remarks.

“All the president said is this is a person who switches identity like a normal person changes their clothes. I think it’s totally proper to call that out,” the Ohio senator said of Trump’s false claims.

“She is not who she pretends to be. She’s flip-flopped on every issue. She’s fake. She’s phony. And I think our whole campaign is going to have a very fun time pointing that out,” Vance said.

Trump’s insensitive racist comments prompted immediate criticism.

Speaking at an event in Houston Wednesday for the Sigma Gamma Rho Sorority, a historically Black sorority, Harris, 59, told her supporters that "this afternoon, Donald Trump spoke at the annual meeting of the National Association of Black Journalists, and it was the same old show."

"The divisiveness and the disrespect, and let me just say, the American people deserve better," she said.

Harris campaign communications director Michael Tyler earlier said the "hostility Trump showed on stage today is the same hostility he has shown throughout his life, throughout his term in office, and throughout his campaign for president."

At the White House, press secretary Karine Jean-Pierre responded to Trump's jab at Harris, calling the former president's comments "insulting."

"As a person of colour, as a Black woman, who is in this position that is standing before you at this podium, behind this lectern - what he just said, what you just read out to me is repulsive, it's insulting," Jean-Pierre said.

Trump has a history of attacking his opponents based on race.

He falsely accused Barack Obama, the country's first black president, of not being born in the US.

Trump attacked the former UN ambassador and his Republican primary opponent Nikki Haley by falsely claiming she could not be president because her parents were not US citizens when she was born.

Harris has faced a series of attacks since becoming the presumptive Democratic nominee. Republicans have criticised the decision, saying she was chosen only because of her race.

Harris has described growing up engaged with her Indian heritage and often visited the country. Her mother also immersed her two daughters in the black culture of Oakland, California - where she was raised, she said.

Trump also attacked Harris' credentials during the discussion, saying she had failed her bar exam early in her legal career.

"I'm just giving you the facts. She didn't pass her bar exam and she didn't think she would pass it and she didn't think she was going to ever pass it and I don't know what happened. Maybe she passed it," he said.

Harris graduated from the University of California Hastings College of Law in 1989. The New York Times reported that she failed her first attempt and passed at the second. The state bar of California says less than half of those who sit the test pass on the first attempt.

Responding to Trump's comments, second gentleman Doug Emhoff said that the former president was “a worse version of an already horrible person” with his remarks.

“The insults..- it’s horrible, it’s terrible, it shows a lack of character - but it’s a distraction,” Emhoff said at a campaign fundraiser in Maine. “It’s about what’s at stake in this election.”