“It’s time to stop fucking around,” Mr. Reiner posted on X yesterday. “If the Convicted Felon wins,” he said, referencing President Trump, “we lose our Democracy. Joe Biden has effectively served US with honor, decency, and dignity. It’s time for Joe Biden to step down.”

As far as shoves out the door go, this one — viewed more than 5.2 million times — was gentle. The reaction of Mr. Reiner’s followers was not. While it’s wise not to overstate the importance of posts on social media, that few of the more than 29,000 comments are supportive indicates that the outrage carries weight.

Mr. Reiner is the son of actor Carl Reiner. The son became a fixture on TV in the 1970s and later transitioned into directing such classic films as “This Is Spinal Tap.” With the advent of social media, he fashioned his fame into a platform for left-wing rage and a bullhorn for its conventional wisdom.

The privileges of wealth and fame mean Mr. Reiner can to speak his mind without fear. Mr. Biden’s loyalists made clear that they have voices, too, and they’re going to use them to shout against friend or foe who rebels against him.

“Stop panicking and show some guts,” a civil rights attorney, Andrew Laufer, replied to Mr. Reiner. “Vote for Joe.” Many said they were unfollowing or even blocking the actor, who on  X dropped overnight to 2.3 million followers from 2.4 million followers.

Others called Mr. Reiner “Meathead,” his nickname on TV’s “All in the Family,” so branded by Archie Bunker because he was “dead from the neck up.” One responder, Resistance Sister, reposted Mr. Reiner’s post-debate tweet, where he cast the choice as between “a good decent man” and “a Convicted Felon who will destroy our Democracy,” and asked what had changed.

In calling for Mr. Biden’s ousting, Mr. Reiner forgot a rule from his sitcom days. Whenever producers changed the actor portraying a character, fans revolted. Take Michael Evans, who portrayed Lionel Jefferson on “All in the Family” and its spinoff, “The Jeffersons.”

Viewers loved Michael Evans, and when he was replaced by Damon Evans, to whom he bore no relation and little resemblance, the new actor was rejected, prompting the original’s return. Audiences are no more favorable to recasting candidates — and thousands of them voted to put Mr. Biden on the stage.

If Mr. Reiner thought his post would be a Nixon-goes-to-China moment rallying others to his banner, it backfired. Only one fellow celebrity jumped out in immediate agreement: Sean Lennon, son of the late singer and one-time Beatle, John, and his wife, Yoko Ono.

“I’m really surprised it took you this long, sir,” Mr. Lennon responded. “It has been obvious for years that Biden is cognitively unfit to serve.” It was backhanded support, and in a thread that gave the impression Mr. Reiner is on an island, it was among the most positive.

The left wing has long cheered Mr. Reiner, who has equated Republicans with Nazis and Trump with Adolf Hitler. That the actor-director came out against Mr. Biden — and faced what the elder Lennon called “Instant Karma” in his song of that name — demonstrates that the president has more staying power than the party sachems seem to think.

After Mr. Biden’s appearance on MSNBC this morning, Mr. Reiner addressed those who think he’s strayed. “If we see the Joe Biden that appeared on Morning Joe today every day until Nov. 5,” he tweeted, “he’ll be able to shut up people like me who think he should step aside.”

Mr. Reiner may wish for someone new at the top of the Democratic ticket, but Mr. Biden’s fans find the idea odious and disloyal. Expect would-be Democratic defectors to take notice of the backlash. A substantial chunk of the party base still bets on their old warhorse, and they’ll revolt against anyone who tries to put him out to pasture.