Trump pointed to a Newsweek column penned by conservative law professor John Eastman, suggesting that Harris is not eligible to serve as vice president.
"I heard it today that she doesn't meet the requirements, and by the way the lawyer who wrote the piece is highly qualified, very talented."For years, Trump promoted the so-called birther conspiracy theory about Barack Obama, suggesting that his predecessor was not born in the United States.
The debunked theory is widely regarded as racist.
Trump made the remarks about Harris just two days after the Democratic Party's presumptive presidential nominee, Joe Biden, announced her as his running mate.
In his op-ed, Eastman argued that the California senator is ineligible to serve as president because her father was a Jamaican national and her mother an Indian national. Neither of the two, he argued, was a naturalized citizen at the time of Harris' birth.
Eastman pointed to the 12th Amendment and Article II of the U.S. Constitution. He also wrote that Harris cannot serve because of the Citizenship Clause of the 14th Amendment.
The law professor concluded that Harris' background needs to be reexamined amid reports of Russian and Chinese election interference, writing that "it would be an inauspicious start for any campaign for the highest offices in the land to ignore the Constitution's eligibility requirements."
Eastman was slammed for writing the column, with critics pointing out that his views on birthright citizenship are not shared by most constitutional law scholars.
As New York Magazine noted, Illinois Sen. Tammy Duckworth was also vetted by Biden's team.
Although a natural-born citizen, Duckworth was not born in the United States, so Democrats reportedly feared that Trump and his allies would attack her with birther conspiracy theories.
"But with another Black nominee on the ballot, Trump is returning to his wheelhouse, questioning the citizenship of a candidate born in the United States," the publication concluded.
In their first joint press conference, Biden and Harris accused Trump of leaving the nation "in tatters."
The duo also slammed Trump's handling of the coronavirus pandemic, with Biden saying that the commander-in-chief spends more time golfing than addressing the unprecedented public health crisis.
Biden talked about Trump's attacks on the California Democrat, suggesting that they do not come as a surprise "because whining is what Donald Trump does best."