US President Donald Trump has said he had heard Democratic vice-presidential running mate Kamala Harris was not eligible to serve, stoking a false claim echoing the baseless "birther" theory he promoted about Barack Obama.

At a White House news conference, Mr Trump was asked about "claims circulating on social media" that Ms Harris was not eligible and whether he could say she met the legal requirements for vice president.

"I heard it today that she doesn't meet the requirements. And, by the way, the lawyer that wrote that piece is a very highly qualified, very talented lawyer. I have no idea if that's right," Mr Trump said.

"I would have assumed the Democrats would have checked that out before she gets chosen to run for vice president," he said.

"But that's a very serious - you're saying that - they're saying that she doesn't qualify because she wasn't born in this country."

The reporter replied that Ms Harris was born in the United States, but her parents might not have been permanent residents at the time.

Mr Trump appeared to be referring to a Newsweek op-ed by a conservative law professor wrongly claiming the California senator was not eligible to serve as vice president or president because of the immigration status of her parents when she was born.

Ms Harris was born in Oakland, California, in 1964 to a father from Jamaica and a mother from India.

Mr Trump earned national political prominence by promoting the "birther" lie that Barack Obama, America's first black president, was not born in the United States.

He grudgingly acknowledged late in his 2016 presidential campaign that Mr Obama was born in the United States.

Who is Joe Biden's running mate Kamala Harris?

In his Newsweek op-ed, Chapman University law professor John Eastman claimed some "commentators" said Ms Harris was ineligible as she was not a "natural born citizen" as her parents were not naturalised US citizens at the time of her birth.

According to the Constitution, any natural born US citizen over the age of 35 is eligible to be president or vice president.

Constitutional law expert Erwin Chemerinsky told CBS News the claim about Ms Harris "is a truly silly argument".

"Under section 1 of the 14th Amendment, anyone born in the United States is a United States citizen. The Supreme Court has held this since the 1890s. Kamala Harris was born in the United States," the dean of UC Berkeley School of Law said in an email to CBS.

"Some conservatives, such as John Eastman, think that is wrong and being born in the country is not enough," he said.

"(They're) clearly wrong under the language of the 14th Amendment and under Supreme Court precedent."

Mr Eastman's op-ed was published a day after Democratic challenger Joe Biden announced Ms Harris as his running mate, the first woman of colour tapped by a major party for the post.

Meanwhile President Trump's aide Jenna Ellis took aim at Kamala Harris by saying she sounded like Marge Simpson, the beloved matriarch of the series The Simpsons.

This provoked a video response today from the famous cartoon character.

"I usually don't get into politics, but the president's senior advisor, Jenna Ellis, just said Kamala Harris sounds like me," said Marge in a video posted on the official Simpsons Twitter account.

The kind-hearted character - known for her majestic blue beehive, long green tube dress and red pearls -  continued: "As an ordinary suburban housewife, I'm starting to feel a little disrespected".

Julie Kavner has voiced Marge for the irreverent show's 31 seasons. The character's description of herself as an "ordinary suburban housewife" is also a swing at President Trump.

The Republican leader recently declared he would win the "suburban housewife" vote in the US election, after he repealed a fair housing regulation implemented under Barack Obama that was intended to prevent housing segregation.