WASHINGTON — President Trump on Thursday encouraged a racist conspiracy theory that is rampant among some of his followers: that Senator Kamala Harris, the presumptive Democratic vice-presidential nominee born in California, was not eligible for the vice presidency or presidency because her parents were immigrants.

That assertion is false. Ms. Harris is eligible to serve.

Mr. Trump, speaking to reporters on Thursday, nevertheless pushed forward with the attack, reminiscent of the lie he perpetrated for years that President Barack Obama was born in Kenya.

“I heard it today that she doesn’t meet the requirements,” Mr. Trump said of Ms. Harris.

“I have no idea if that’s right,” he added. “I would have thought, I would have assumed, that the Democrats would have checked that out before she gets chosen to run for vice president.”

Mr. Trump appeared to be referring to a widely discredited op-ed article published in Newsweek by John C. Eastman, a conservative lawyer who has long argued that the United States Constitution does not grant birthright citizenship. Ms. Harris, the daughter of Jamaican and Indian immigrants, was born in 1964 in Oakland, Calif., several years after her parents arrived in the United States.

But Mr. Trump was in effect revisiting an old tactic: spreading a race-based and anti-immigrant crusade he began nearly a decade ago, when he began sowing distrust in the background of Mr. Obama, who was born in Hawaii.

This time, Mr. Trump has legions of followers who have been spreading similar theories about Ms. Harris. In the hours after Joseph R. Biden Jr. announced Ms. Harris as his running mate, a new crop of memes and conspiracy website postings began proliferating online, suggesting that Ms. Harris was an “anchor baby,” a disparaging term for a child born in the United States to immigrants.

Mr. Eastman’s column tries to raise questions about the citizenship of Ms. Harris’s parents at the time of her birth, and argues that she may have “owed her allegiance to a foreign power or powers” if her parents were “temporary visitors” and not residents. Ms. Harris’s parents received doctorate degrees from the University of California, Berkeley, in 1963 and were working as academics when Ms. Harris was born in 1964.

But constitutional law scholars say that the immigration status of Ms. Harris’s parents at the time of her birth is irrelevant because under the Constitution, anyone born in the United States automatically acquires citizenship.

The 14th Amendment makes it clear: “All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside.”

Nonetheless, Mr. Eastman’s article leapfrogged throughout social media on Thursday. Tom Fitton, the president of the conservative group Judicial Watch — a favorite information source of Mr. Trump’s — shared the article on Twitter. By Thursday afternoon, it had reached some 14.3 million people on Facebook, Reddit and Twitter before it was parroted by the president, according to data reviewed by The New York Times.

Newsweek in the meantime defended Mr. Eastman’s column, asserting that it had “nothing to do with racist birtherism.” Experts in constitutional law were still quick to disparage the article as dangerous.

In an interview on Thursday, Laurence H. Tribe, a professor of constitutional law at Harvard Law School, compared Mr. Eastman’s idea to the “flat earth theory” and called it “total B.S.”

“I hadn’t wanted to comment on this because it’s such an idiotic theory,” Mr. Tribe said, “There is nothing to it.”

Mr. Tribe pointed out that the theory still quickly landed in the hands of a president who has used his pulpit to spread a number of conspiracies against his political enemies, particularly those who do not have white or European backgrounds.

During the 2016 presidential race, Mr. Trump continuously questioned the citizenship of Senator Ted Cruz of Texas, suggesting that his Canadian roots would be a problem should he win the presidency. Mr. Cruz, who was born in Canada to an American mother and a Cuban father, is a United States citizen. Mr. Eastman, for his part, wrote that year that Mr. Cruz was eligible.

But Mr. Trump was relentless about questioning Mr. Obama’s background. In 2011, he began appearing on television to question whether Mr. Obama was born in the United States — spreading the lie he has never fully apologized for.

“Maybe I’m going to do the tax returns when Obama does his birth certificate,” he said in an ABC interview in April 2011. “I’d love to give my tax returns. I may tie my tax returns into Obama’s birth certificate.”

Mr. Obama eventually released his birth certificate. Mr. Trump has never released his tax returns.

At the White House Correspondents Dinner in 2011, Mr. Obama acknowledged that he released his long-form birth certificate, and took aim at Mr. Trump, who was sitting in the audience.

“He can finally get back to focusing on the issues that matter, like, did we fake the moon landing?” Mr. Obama said as a stone-faced Mr. Trump looked on. He also displayed a rendering of the White House, styled as a casino, should Mr. Trump win the presidency.

Mr. Trump, of course, ended up running and winning. In 2016, he finally, and tersely, acknowledged that Mr. Obama was an American citizen.

“President Barack Obama was born in the United States, period,” Mr. Trump said at the time. “Now, we all want to get back to making America strong and great again.”

He then falsely suggested that Hillary Clinton, his former Democratic opponent, had started the rumor.

Ben Decker contributed reporting.