President Biden’s campaign, after more than a year of studiously avoiding direct comments about Donald J. Trump’s legal problems, on Tuesday dispatched the actor Robert De Niro to the Manhattan courthouse where the former president’s first criminal trial is wrapping up.
Mr. De Niro — in an off-script moment — declared that Mr. Trump was guilty and should go to jail.
“The fact is whether he’s acquitted, whether it’s hung jury, whatever it is, he is guilty, and we all know it,” the actor said after the news conference, as a rowdy group of Trump supporters confronted him and yelled insults before he left. “I’ve never seen a guy get out of so many things, and we all know this. Everybody in the world knows this.”
Asked if he thought Mr. Trump should be in jail, Mr. De Niro replied: “I sure do. Absolutely.”
The foray by a Biden surrogate into commentary about Mr. Trump’s guilt, which the New York jury may decide as soon as this week, was a stark departure from the president’s directive to avoid discussing his rival’s felony charges. Mr. Biden has said next to nothing on the subject, to avoid feeding the false Trump-inspired narrative that he ordered prosecutors to bring criminal charges against his predecessor.
The moment illustrated how difficult it may be for the Biden campaign to navigate its response to a potential verdict, with outside allies far more willing than his disciplined operation in Wilmington, Del., to lob frontal attacks at Mr. Trump over his legal peril.
The news conference’s intended focus was to draw attention to Mr. Trump’s actions leading up to the Jan. 6, 2021, attack on the Capitol, which are the subject of another federal criminal case pending against him. In Manhattan, Mr. Trump is charged with falsifying business records related to a hush-money payment to a porn star before the 2016 election.
Mr. De Niro, whose voice narrates the campaign’s latest ad, appeared alongside Harry Dunn and Michael Fanone, two former U.S. Capitol Police officers who have criticized Mr. Trump over his role in the Jan. 6 mob violence.
Mr. Fanone accused Mr. Trump of being “an authoritarian who answers only to himself,” and Mr. De Niro said the former president had engaged in a “coward’s violence” after the 2020 election.
“He directs the mob to do his dirty work for him,” he said.
Mr. De Niro also mentioned the civil case last year in which a jury found Mr. Trump liable for sexually abusing and defaming E. Jean Carroll, a former magazine writer. In January, the former president was ordered to pay $83.3 million in damages to Ms. Carroll. The Biden campaign has rarely discussed that case or the verdict against Mr. Trump.
“Just a couple of blocks from here a jury found him liable for sexual abuse,” Mr. De Niro said.
The Trump campaign quickly jumped on the Biden news conference, and Mr. Trump shared a video on social media of Mr. De Niro yelling at protesters.
“The Biden folks have finally done it,” said Jason Miller, a senior adviser to the campaign. “After months of saying that politics had nothing to do with this trial, they showed up and made it with a campaign event.”
He characterized Mr. De Niro’s involvement as a desperate reach for a campaign that needed to lift Mr. Biden’s poll numbers, referring to him as a “washed-up actor” and arguing that Biden campaign staff members at the news conference had sensed it was a bad idea to stand in the proverbial shadow of the courthouse.
The Biden campaign said that bringing Mr. De Niro and the two Capitol Police officers to Mr. Trump’s trial was not a commentary on the case but on the news media scrum gathered to cover it.
“We’re not here today because of what’s going on over there,” Michael Tyler, the campaign’s communications director, told the reporters assembled outside the courthouse. “We’re here today because you all are here.”
Throughout the trial, many of the news conferences outside the courthouse have been met with some level of heckling from demonstrators stationed in a protest area nearby. Pro-Trump demonstrators shouted at Mr. De Niro and the former Capitol Police officers, calling the actor an elitist and the officers traitors.
At one point, Mr. De Niro flashed anger reminiscent of one of his movie characters.
“We’re trying to be gentlemen, the Democrats,” he yelled at the Trump supporters. “You are gangsters — you are gangsters!”
Mr. Trump has sought to tie together all four of his pending criminal cases and has argued baselessly that Mr. Biden is behind them all. In addition to the Manhattan trial, he is charged in separate federal cases over his handling of classified documents and his efforts to overturn the 2020 election, along with a Georgia case related to his push to reverse that year’s results.
Steven Cheung, a Trump campaign spokesman, said the Biden campaign was “making a political mockery” of the criminal case with its appearance.
Asked about the Biden campaign’s contention that it had held a news conference at the courthouse only because the national news media had been stationed there for weeks, Karoline Leavitt, the Trump campaign’s press secretary, responded simply: “They’re pathetic.”
Later on Tuesday, Mr. Trump’s two oldest sons and his daughter-in-law Lara Trump held the a news conference of their own outside the courthouse.
Ms. Trump used the opportunity to try to raise money. “If they can profit off it, on the other side, so can we: donaldjtrump.com,” she said.
Donald Trump Jr. also took a potshot at Mr. De Niro.
“He needs attention because it’s been a while since he cranked out a good movie,” the younger Mr. Trump said. “The fact that they are holding a rally right across the street from this very witch hunt, right across the street, tells us exactly what we all knew all along, which is that it is a political persecution.”
As Mr. De Niro walked away from his downtown appearance, trailed by jeers from Trump supporters, he offered them an angry response that was more in keeping with the Biden campaign’s overall message.
“Screaming and yelling and intimidating, you’re not going to intimidate, that’s what Trump does, to try to intimidate,” Mr. DeNiro shouted, jutting out a finger. “We are going to fight back.”