Using Cartoonish Accents, J.F.K.’s Grandson Insults and Mocks Robert F. Kennedy Jr.

The grandson of President John F. Kennedy this week savaged his presidential-candidate cousin, Robert F. Kennedy Jr., in a series of mocking, meant-to-be-funny videos that were, inarguably, uncharacteristically un-Kennedyesque, escalating a civil war within America’s most storied political dynasty.

In a series of Instagram posts, the grandson, Jack Schlossberg, 31, variously called Mr. Kennedy, 70, a “prick,” suggested he was using steroids, said he was “lying to us” and portrayed him as a Russian stooge and a stalking horse for Donald J. Trump.

But what viewers may be more struck by, or even insulted by, are the heavily accented caricatures the young scion used to dramatize his points.

He impersonates a Massachusetts fan of the Kennedys named Jimmy, sounding like Ben Affleck in a Dunkin’ Donuts commercial.

“You know, I’m a fan of his father,” Mr. Schlossberg says, as Jimmy. “And you know his uncle? Rest in peace, I remember where I was the day he was killed, I mean it was a tragic day, the entire country wept. But listen, that guy, he’s a prick. The new guy, the young guy, he’s a friggin prick.”

He channels a southerner named Wade who raises horses and says, “You can always tell when a horse is being pumped full of testosterone — steroids doesn’t make the horse think any better.” And he conjures an Italian-American Long Islander named Anthony (or “Ant’ny”) who worries that Mr. Kennedy would slash the military at a time when China, Russia, “everyone’s on our ass.”

What may be the edgiest impression, though, is Mr. Schlossberg’s depiction of Joshua, an older New York Jewish man, who speaks in a kindly, singsong voice about Mr. Kennedy, until coming around to his fear that the candidate would follow through on his promise to do away with the Federal Reserve.

If so, he shouts, “Who is going to protect my money?”

For a family whose chowder-specked, “r”-averse New England timbre has long been the subject of hacky impressions from the public, Mr. Schlossberg’s wheel of faux-voter imitations was a striking inversion: While so many were imitating the Kennedys, this Kennedy, it appears, was working on his impressions of so many.

Here was a prince of Camelot, John Bouvier Kennedy Schlossberg, with possible political ambitions of his own, cycling between stereotypes of workaday voters to diminish a relative thought to be damaging the Kennedy name.

Mr. Schlossberg has long been in Mr. Biden’s corner. He was featured at the 2020 Democratic National Convention alongside his mother, Caroline Kennedy, who is now Mr. Biden’s ambassador to Australia. And in a social-media video in July, he called his cousin’s presidential campaign, then aimed at defeating Mr. Biden in the Democratic primary, “an embarrassment.”

But this attack seemed more stinging.

Some online fans have cheered the online antics of the young Kennedy heir, who has three Ivy League degrees — and apparently abundant leisure time: Bearing a striking resemblance to his late uncle, John F. Kennedy Jr., Mr. Schlossberg has posed flexing on the beach, breaking open coconuts, surfing and otherwise playing the role of a laid-back beach bum.

The Biden campaign did not immediately respond to questions of whether Mr. Schlossberg’s video barrage had been coordinated with the campaign.

It did, however, seem likely to further strain his extended family.

Mr. Kennedy, whose campaign did not respond to a request for comment, has leaned heavily on throwback imagery and Camelot nostalgia, offering himself as an heir to the family’s political legacy.

Mr. Schlossberg’s videos — which have often appeared to be filmed beachside in a tropical-looking locale — are a very modern reminder that broad swaths of the Kennedy family wholly reject that idea.

At minimum, Mr. Schlossberg, who has hinted in the past that he has his eye on elected office, has demonstrated a willingness to take comedic risks. Then again, candidate humor — itself an occasional oxymoron — tends to succeed most when it is self-deprecating, rather than aimed at members of the electorate.

Whatever his flaws, and however reliant he may be on hackneyed stereotypes, Mr. Schlossberg is plainly a committed performer.

And someone who pays attention to his replies.

Seeing the reaction to his videos, as well as some criticism from people taking offense to his caricatures, he posted a series of sorry-not-sorry clips, again in character.

“They say I do these accents, I’m making fun of people, making fun of working people,” he said as Anthony. “Not my intention, no disrespect.” And as Joshua, he insisted that he was “not making fun of anyone, because I am Jewish, and I grew up in New York City.”

“These characters,” he added — still in Joshua’s voice, but seeming to speak now as himself — “actually help me cope.”