“I would hope you not repeat that,” Trump said sarcastically according to an attendee.

With Biden emerging as the likely Democratic nominee, Trump has launched a concerted, near-daily campaign to raise doubts about the 77-year-old’s mental acuity. The president has been bolstered by a conservative echo chamber flooding social media with video clips highlighting Biden’s gaffes.

The effort provides a window into how Trump — who’s been dogged by questions about his own mental fitness — regularly picks apart his political opponents. He has an unmatched ability to zero in on his foe’s biggest vulnerability or insecurity, and through sheer repetition bake it into the public consciousness. Unfortunately for Biden, his performance on the campaign trail has given Trump plenty to work with.

It’s similar to the tack Trump used in 2016 against Hillary Clinton, whom he tagged as “low energy.” He also suggested she “doesn’t have the stamina” to be president.

Former Clinton advisers see a replay of that campaign, and warn that Biden needs to take the attacks seriously.

“He’s not responding to the threat strong enough, because it is absolutely a problem now and [is] going to be a problem” going forward, said Philippe Reines, a former top Hillary Clinton adviser who prepped her for the debates with Trump. “You have to defend yourself, because that stuff absolutely sticks.”

Trump has long branded Biden as “Sleepy Joe.” But in the week following Biden’s win in the South Carolina Democratic primary, Trump has taken it a step further by questioning the former vice president’s mental faculties. The president took to Twitter to say that Biden “doesn’t know where he is, or what he’s doing,” and said Biden would destroy entitlement programs "and he won’t even know he’s doing it!"

WOW! Sleepy Joe doesn’t know where he is, or what he’s doing. Honestly, I don’t think he even knows what office he's running for! pic.twitter.com/454v9GFpgF

— Donald J. Trump (@realDonaldTrump) March 3, 2020

The president went at Biden again during a rally in Charlotte, N.C. last week, saying that once elected, Biden would be put “into a home and other people are going to be running the country.”

Then, during a Fox News town hall Thursday evening, Trump pointed to a string of recent Biden gaffes, including the former vice president’s remark that Super Tuesday was taking place on “Super Thursday.”

“There's something going on there,” Trump said.

Trump is getting backup from his sprawling political apparatus. The Republican National Committee last week circulated a clip showing Biden appearing to confuse his wife and his sister, distributed social media posts making fun of the former vice president for his Super Thursday remark, and cut a video depicting Biden botching a line from the Declaration of Independence.

Some of the president’s conservative allies are taking the attack even further. During a Saturday appearance on Fox News, Trump attorney Rudy Giuliani said Biden is showing “obvious signs of dementia.” Fox News host Sean Hannity, meanwhile, devoted an entire segment of his show to documenting how Biden has “a very questionable grip on reality.”

Tucker Carlson, another Trump-friendly Fox News host, said Biden is "clearly losing it."

Trump allies say the president’s attacks are damaging because he homes in on vulnerabilities in a way that sticks with voters.

“The president is expert at indelibly tagging his opponents with nicknames that resonate in flyover country. He is similarly adept at identifying and highlighting his opponents’ shortcomings in unavoidable and ubiquitous commentary,” said Michael Caputo, a Trump 2016 campaign aide.

With Trump’s “constant reminders,” Caputo added, “voters won’t be able to look away from Biden’s guaranteed gaffes, and this will amplify existing concerns to weigh down the former vice president.”

Trump campaign officials said the president is merely highlighting the mistakes Biden is making.

“Joe Biden was a terrible candidate the first two times he ran for president,” said Tim Murtaugh, a spokesman for the Trump reelection effort. “I don’t know why anyone expected he would be better the third time around.”

Biden has laughed off the attacks. During a recent appearance on Fox News Sunday, the former vice president was asked to respond to a clip of Trump saying that he would “be sitting in a home someplace” if elected.

“Is that the stable genius saying that?” Biden shot back.

Those close to the Democratic frontrunner dismiss the attacks, saying that voters see Biden’s verbal hiccups as part of his charm. The assault, they argue, reflects the profound degree to which Trump is threatened by the former vice president. Trump was impeached by the Democratic-led House last year over allegations that he pressured the Ukrainian government into investigating Biden and his family.

“With the Democratic Party galvanized by the best candidate to win crucial battleground states, it’s not surprising that Trump’s reelection campaign is resorting to more disinformation about the vice president — after all, Trump himself is so panicked about Joe Biden that he got himself impeached by trying to force a foreign country to lie about him,” said Andrew Bates, a Biden campaign spokesman.

The attacks on Biden bear a striking resemblance to the ones Trump launched against Clinton four years ago. In 2016, Trump was similarly echoed by an array of conservative allies. The pro-Trump Breitbart News published a series of articles about Clinton’s health, including one that linked to a story on a conservative website alleging the former secretary of State was engaging in “bizarre behavior” and questioned whether she had a seizure.

Infowars, another pro-Trump site that peddles in conspiracy theories, piled on with a piece exploring whether Clinton had Parkinson’s disease.

The onslaught only intensified after Clinton suffered a bout of pneumonia while attending a Sept. 11 memorial event.

Clinton allies said the attacks took a toll. Reines urged Biden to respond forcefully by raising questions about Trump’s mental fitness and about why the president hasn’t been more forthcoming about his health.

“It’s an absolute layup,” Reines said. “And the vice president, bless his soul, doesn’t seem to take to … what he probably thinks is mean or adversarial. But, ‘This is the business we’ve chosen,’ as The Godfather said.”

Other Democrats, however, aren’t so sure Trump’s offensive will work.

Jennifer Palmieri, who served as communications director on Clinton's campaign, pointed out that Trump has been savaging Biden for months with questionable results. Democratic and independent voters, she said, were tuning out the attacks.

“The Trump campaign has been hitting Biden and his family for over a year and the attacks don’t seem to weaken him with voters,” Palmieri said. “He just gets stronger.”