CHICAGO (AP) — As former Vice President Joe Biden’s drive for the White House gains momentum, the 77-year-old’s political opponents on both the right and left have launched an internet campaign suggesting he’s not mentally or physically equipped to serve — sometimes using altered content and other disinformation to make their case.

The effort prompted Twitter on Monday to mark a video of Biden shared by President Donald Trump as “manipulated media” — but only after it had been viewed millions of times. Facebook added warning labels to the video and reduced its distribution on its platform after news organizations fact checked the video on its site. The Associated Press works with Facebook as a third-party fact checker.

The episode highlights the challenges social media companies face in containing the rapid spread of misleading information and raises questions about how prepared the companies are for a likely onslaught of disinformation during the 2020 campaign. It also underscores that trafficking in misinformation is not limited to one political party: Both Sanders' and Trump's backers aggressively promoted the unsubstantiated notion that Biden is in cognitive decline.

“This is the worst kind of misinformation — it can impact a presidential election, and it's being shared by the president with millions of followers,” said Jennifer Grygiel, a Syracuse University communications professor who studies disinformation and social media. “We're going to see a lot more of it, and we need the platforms to get this right.”

All in their 70s, Biden, Sanders and Trump have faced questions about their stamina or mental acumen, though each has produced letters from physicians attesting to his fitness for office. Sanders — who, like Biden, would be the oldest president ever elected — suffered a heart attack last year. Biden has long had a penchant for verbal miscues throughout his political career. And Trump has directly tried to combat questions about his mental fitness, tweeting in 2018 that he is a “very stable genius.”

The unproven claims about Biden’s fitness for office have emerged from the left and right with increasing frequency following the former vice president’s sweeping victories during last week’s Super Tuesday contests. And they accelerated after Biden returned to the campaign trail on Saturday with a brief speech in St. Louis.

Sanders’ campaign manager, Faiz Shakir, pointedly noted on Twitter that his 78-year-old boss had three campaign events on his schedule that day, “each speaking engagement extending for close to an hour.”

Later in the day, Trump tweeted a misleadingly edited video clip from Biden’s 15-minute stump speech in Kansas City, Missouri.

During the speech, Biden appeared to stumble over his words when he said: “We cannot win this reelection -- excuse me, we can only reelect Donald Trump if, in fact, we get engaged in this circular firing squad here."

Conservative and pro-Trump social media accounts first seized on the gaffe Saturday evening, sharing a fuller clip. But White House social media director Dan Scavino later shared a further edited version in which Biden appears to say: “We can only reelect Donald Trump.” Trump retweeted Scavino's post.

Different versions of the video were quickly shared by Twitter accounts affiliated with Trump's campaign Saturday night over a three-hour period, noted Gideon Blocq, the CEO of VineSight, a tech company that surfaces misinformation online.

Combined, the various versions of the edited clip were watched more than 10 million times by Monday afternoon.

The Biden campaign responded to the video with incredulity and noted that the former vice president has kept up a whirlwind schedule in recent weeks. The supposed evidence of his decline, they noted, comes from public appearances with voters and reporters present.

“It's revealing that Donald Trump and his campaign keep admitting, through their reliance on desperate smears, that they can’t beat Joe Biden with the truth,” said Biden spokesman Andrew Bates.

Bates added a dig at Sanders supporters who have echoed many of the Republican attacks on Biden. “Any 'Democrat' who aligns themselves with Trump by amplifying his disinformation,” Bates said, “will soon learn that they can’t beat the vice president with wild-eyed lies, either."

Identifying, labeling and removing false claims or edited videos is emerging as a key challenge to tech platforms that have been criticized on both sides of the political spectrum.

Twitter’s decision to mark the Biden video as “manipulated media” was the first test of a new policy of labeling and — in some cases removing — manipulated images designed to mislead. But even that effort failed to take hold for some social media users because of a technical error, a Twitter spokesman said Monday. It declined to say whether it had labeled other versions of the edited video.

Trump allies blasted Twitter's decision to label the edited video as “manipulated.”

Trump legal adviser Jenna Ellis called criticism of the edited video, which she also shared, “insanely dishonest” on Twitter. She said people are missing the point: ”The point is the gaffe. DUH.”

Liberal and pro-Sanders social media accounts have been promoting the idea that Biden is in “cognitive decline” on YouTube, Facebook and Twitter in recent days.

A two-minute video montage of Biden’s gaffes on the campaign trail titled “Joe Biden’s sharp cognitive decline (please share, voters need to know)” has been viewed nearly 300,000 times since it was posted to a “populist” news and commentary YouTube page on Saturday.

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Klepper reported from Providence, R.I. Associated Press writer Bill Barrow in Atlanta contributed to this report.

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