President Joe Biden exited the race on Sunday and endorsed Harris -- the first Black, South Asian and woman vice president in US history -- who vowed to win her party's nomination to take on Donald Trump in November.
An online explosion of misogynistic and sexist narratives about Harris quickly ensued, including previously debunked falsehoods.
Some social media posts repeated suggestions Harris "slept her way to the top" in American politics, citing her brief relationship in the 1990s with former San Francisco mayor Willie Brown.
The charge was refloated by conservative influencers such as Candace Owens, Matt Walsh, and Clay Travis.
Meanwhile, posts on the platform X recirculated a doctored image of Harris appearing to pose alongside disgraced financier and convicted sex offender Jeffrey Epstein.
The picture -- debunked years ago by AFP's fact-checkers using a reverse image search -- had been manipulated to include Epstein instead of Harris's husband, Douglas Emhoff.
Online posts also derided US-born Harris as a "Black African woman," with some attributing her success solely to her ethnicity.
"It's important to label these narratives and lies as what they are: an attempt to undermine a powerful woman's public service because of her gender, her background, her skin color," said Nina Jankowicz, co-founder of the disinformation watchdog American Sunlight Project.
"I challenge anyone who opposes Harris's candidacy to engage in a substantive debate on the merits of her policies and track record, rather than calling her disgusting names."
- 'Lies and conspiracies'
In 2020, Jankowicz led a study that found more than 336,000 instances of "gendered abuse and disinformation" used to attack 13 women politicians. Some 78 percent of that targeted Harris.
The disinformation involved not just sexual tropes but also false transphobic narratives, such as Harris could not have ascended politically without secretly being a man.
Also included were racist narratives falsely asserting Harris was "ineligible" to run for office because both her parents were immigrants, while some insisted that she was "exaggerating" her racial identities for political gain.
Roberta Braga, executive director of the Digital Democracy Institute of the Americas, warned internet users to be alert for "lies and conspiracies" about Harris in the coming days.
"The misinformation will be laced with gender-based attacks. And it won't be new," Braga said.
Women candidates of color in the 2020 elections were twice as likely as other candidates to be targeted with disinformation, according to a report from the Washington-based Center for Democracy & Technology.
They were also four times as likely as white candidates to be targeted with violent abuse, the report added.
'Full spectrum'
Gendered disinformation –- when sexism and misogyny intersect with online falsehoods -- has relentlessly targeted women politicians around the world, tarnishing their reputations, undermining their credibility and, in many cases, upending their careers.
AFP's global fact-checkers have regularly debunked falsehoods targeting politically active women, who are often sitting ducks for online abuse and sexually-charged trolling.
As the White House race -- already vulnerable to an avalanche of disinformation -- heads into its final months, researchers are bracing for a flood of falsehoods targeting Harris.
Widely available artificial intelligence tools are expected to add fuel to the fire on social sites such as X -– the platform formerly known as Twitter and owned by Elon Musk, who is strongly backing Trump.
Platforms including X have scaled back content moderation, removing many of the guardrails against false information, and reinstated accounts of known purveyors of falsehoods.
"We should expect a full spectrum of disinformation," said Ronald Deibert, director of the Citizen Lab, a research center affiliated with the University of Toronto.
That will range from "well-organized and professional influence operations, in some cases backed by foreign adversaries, through to amateur productions created by miscreants," Deibert told AFP.
