President Biden on Friday delivered a ferocious condemnation of Donald J. Trump, his likely 2024 opponent, warning in searing language that the former president had directed an insurrection and would aim to undo the nation’s bedrock democracy if he returned to power.On the eve of the third anniversary of the Jan. 6, 2021, attack on the Capitol by Mr. Trump’s supporters, Mr. Biden framed the coming election as a choice between a candidate devoted to upholding America’s centuries-old ideals and a chaos agent willing to discard them for his personal benefit.“There’s no confusion about who Trump is or what he intends to do,” Mr. Biden warned in a speech at a community college not far from Valley Forge in Pennsylvania, where George Washington commanded troops during the Revolutionary War. Exhorting supporters to prepare to vote this fall, he said: “We all know who Donald Trump is. The question is: Who are we?”In an intensely personal address that at one point nearly led Mr. Biden to curse Mr. Trump by name, the president compared his rival to foreign autocrats who rule by fiat and lies. He said Mr. Trump had failed the basic test of American leaders, to trust the people to choose their elected officials and abide by their decisions.“We must be clear,” Mr. Biden said. “Democracy is on the ballot. Your freedom is on the ballot.”The harshness of Mr. Biden’s attack on his rival illustrated both what his campaign believes to be the stakes of the 2024 election and his perilous political standing. Confronted with low approval ratings, bad head-to-head polling against Mr. Trump, worries about his age and lingering unease with the economy, Mr. Biden is turning increasingly to the figure who has proved to be Democrats’ single best motivator.Mr. Trump, speaking at a campaign rally in Iowa soon after Mr. Biden’s appearance, quickly lashed back, calling the president’s comments “pathetic fearmongering” and accusing him of “abusing George Washington’s legacy.”Mr. Biden’s remarks carried echoes of the 2020 campaign, when he presented himself as the caretaker of “the soul of America” against a Trump presidency that he and Democratic supporters argued was on the verge of causing permanent damage to the country.The 31-minute speech was Mr. Biden’s first public campaign event since he announced in April that he would seek re-election and was, in tone and content, arguably his most forceful public denunciation of Mr. Trump since the two men became political rivals in 2019.Mr. Biden’s appearance, meant as a kickoff to help define the 2024 campaign, was an early effort to revive the politically sprawling anti-Trump coalition that propelled Democrats to key victories in recent elections. Mr. Biden’s task now is to persuade those voters to view the 2024 contest as the same kind of national emergency that they sensed in 2018, 2020 and 2022.President Biden and Jill Biden, the first lady, participated in a wreath ceremony at the Valley Forge National Arch before his speech on Friday. Pete Marovich for The New York TimesHe began with an extensive recounting of Mr. Trump’s actions before, during and after the Jan. 6 attack. The country, Mr. Biden said, cannot afford to allow Mr. Trump and his supporters to present a whitewashed version of that day and spread falsehoods about the violent outcome of their effort to undo the 2020 election results. Upholding the nation’s democracy, Mr. Biden said, is “the central cause of my presidency.”Mr. Biden said that, by contrast, Mr. Trump “refuses to denounce political violence,” asserting, “You can’t be pro-insurrectionist and pro-American.”Mr. Trump and his allies have spent the three years since the Capitol riot denying and deflecting his responsibility, downplaying the seriousness of the bloodshed and going so far as to suggest it was all a plot by Mr. Biden’s allies deep within the federal government to make Mr. Trump look bad.“Trump is trying to steal history, the same way he tried to steal the election,” Mr. Biden said. “It was on television. We saw it with our own eyes.”Mr. Biden made no mention of the 91 felony charges the former president faces in four jurisdictions, sticking to a vow to steer clear of his rival’s legal problems and focusing squarely on Mr. Trump’s actions rather than any potential criminal consequences for them.“Trump exhausted every legal avenue available to him to overturn the 2020 election. The legal path took him back to the truth, that I won the election and he was a loser,” Mr. Biden said. “He had one act left, one desperate act available to him, the violence of Jan. 6.”For a president who has faced intense scrutiny over his vigor in public appearances, the speech was a deftly delivered, focused argument about this year’s stakes. It was Mr. Biden’s latest attempt to build his political identity around the ideas of restoring national unity and upholding fairness, democracy and collective patriotism.He has come back to those themes many times, during his brief push for voting rights legislation in early 2022, then as the midterm elections approached and most recently in September, during a speech in Arizona honoring former Senator John McCain.On Friday, Mr. Biden sought to frame Mr. Trump as the leader of a cult of personality, and his Republican allies as sycophants. The president mentioned the recent $148 million judgment against Rudolph W. Giuliani for his lies about Georgia election workers, as well as the $787.5 million that Fox News was ordered to pay to settle a defamation case about its role in spreading election lies.Mr. Biden lamented that Fox News hosts and Republican officials who condemned Mr. Trump’s Jan. 6 behavior in the moment had since changed their tune and repeated his falsehoods.“Politics, fear and money all intervened, and now these MAGA voices who know the truth about Jan. 6 have abandoned democracy,” Mr. Biden said.But what remains unclear is how much Mr. Biden’s democracy pitch will resonate with voters who remain nervous about an improving economy, and wary of re-electing an 81-year-old who is already the oldest president in U.S. history.Even some who have expressed deep fears about Mr. Trump’s authoritarian impulses are skeptical that the subject will be a winning message in 2024.“As a Biden campaign theme, I think the threat to democracy pitch is a bust,” Senator Mitt Romney of Utah wrote in a text message to a New York Times reporter. “Jan. 6 will be four years old by the election. People have processed it, one way or another. Biden needs fresh material, a new attack, rather than kicking a dead political horse.”Mr. Biden and his campaign have often sought to remind voters of the violence of Jan. 6, 2021. Jason Andrew for The New York TimesDemocrats have found that while ideas of democracy can motivate the party’s most engaged voters, it can be more of a struggle to connect lofty ideals to voters who are more focused on economic issues like high prices and interest rates. In the 2022 midterm elections, months after the fall of Roe v. Wade, far more Democratic candidates made abortion rights central to their messaging as opposed to threats to democracy.Rosa Brooks, a law professor at Georgetown University who convened planning sessions in 2020 to prepare for ways the Trump administration could disrupt that year’s election, said she was worried that “we’re in the nothing-matters phase of American politics.” Mr. Trump’s supporters, she lamented, become only more loyal each time he does something that in a previous era would have been instantly disqualifying.“It’s not clear to me that anything Biden does could fundamentally change any of that,” Ms. Brooks said. “So I’m actually quite depressed.”The Democratic governor of the state Mr. Biden was visiting, Josh Shapiro, who won office in 2022 against an election denier who chartered buses to Washington on Jan. 6, said before Mr. Biden’s speech that the key for the president and fellow Democrats would be connecting the idea of democracy with bread-and-butter issues like health care and the economy. A return to power by Mr. Trump, he said, would “create chaos” across a spectrum of issues that would affect people.“He brought real chaos to this country, and we should not allow that to come back,” Mr. Shapiro said.Senator John Fetterman of Pennsylvania was more succinct. “I see ’24 as good versus evil,” he said.Mr. Biden threaded his speech with warnings that Mr. Trump and Republicans would threaten not only democracy but also major Democratic priorities — abortion rights, voting rights and economic and environmental justice.Ian Bassin, the executive director of Protect Democracy, a nonprofit dedicated to combating authoritarianism, said he had stressed to Mr. Biden’s aides that the president needed to connect democracy to voters’ personal experiences on other issues, in the same way Mr. Trump repeats to his supporters that prosecutions of him are persecutions of them.“Democracy is not just a way of structuring elections for order in our government,” Mr. Bassin said. “It’s a set of values about the kind of communities we want to live in and the way that we want to live as neighbors.”Mr. Biden warned in his speech that Mr. Trump was not being shy about what he would do in a second term.“Trump’s assault on democracy isn’t just part of his past. It’s what he’s promising for the future,” Mr. Biden said. “He’s not hiding the ball.”Mr. Biden then recounted, in exacting detail, how a Trump campaign rally last year began with a choir of rioters who stormed the Capitol on Jan. 6 singing the national anthem while a video of the damage played on a big screen. Mr. Trump had watched with approval.The scene, Mr. Biden suggested, would be the nation’s fate if Mr. Trump and his allies returned to power.“This is like something out of a fairy tale,” Mr. Biden said. “A bad fairy tale.”Kellen Browning More