“The most hated man in Hollywood” — that’s what some internet critics are calling Oscar-winning actor Robert De Niro following his rambling monologue on Tuesday outside the Manhattan courthouse where closing arguments were being made at President Trump’s first criminal trial.

Mr. De Niro is at risk of losing “millions of fans,” critics are saying on X, amid the fallout to his appearance at a press conference organized by President Biden’s campaign. In his speech, the star made several far-fetched claims about the 45th president, asserting that “eventually he could destroy the whole world” and that elections will be “over” if he gets into office, because “he will never leave.”

Now, some are vowing to never again watch the films starring Mr. De Niro, like “The Godfather.” “An iconic actor’s legacy destroyed by a terminal case of TDS,” wrote one commentator on X, referencing “Trump derangement syndrome,” a term used by Trump supporters to describe irrational criticism of him. 

The backlash underscores the growing struggles of the media team on Mr. Biden’s reelection campaign as the president flounders in the polls. “It’s embarrassing that even Vito Corleone AKA De Niro,” one commentator remarked in a reference to “The Godfather,” “can’t win New Yorkers over.”

Pro-Trump demonstrators attempted to drown out the 80-year-old actor’s remarks on Tuesday with chants like “De Niro is a gangster” and accusations that he is “a paid actor for the DNC.” He shouted back, “You’re a f***king idiot.” It seems they are imploring Mr. De Niro to take the advice of his character, Jimmy Conway, in the film “Goodfellas”: “always keep your mouth shut.”

That appearance marked the latest show of support for the president from Mr. De Niro, a far cry from Conway. In a 30-second advertisement for Mr. Biden that will air on television and digital platforms across battleground states, the actor says in an ominous tone, “Trump wants revenge, and he’ll stop and nothing to get it.”

A host of other celebrities are publicly backing Mr. Biden. Actress Mindy Kaling hosted a star-studded fundraiser for him at New York City in March that featured singer Lizzo, talk show host Stephen Colbert, and actor Ben Platt.