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.”