Former Vice President Joseph R. Biden Jr. announced Thursday that he would seek the Democratic nomination to challenge President Trump in 2020, casting the election as a national emergency and asking Democrats to put the task of defeating Mr. Trump above all their other ambitions.
In a three-and-a-half minute video that focused on excoriating Mr. Trump, Mr. Biden presented himself as a steely leader for a country wracked by political conflict. Unlike the wide field of Democrats competing for the affections of the left, Mr. Biden avoided almost any talk of policy or ideology, signaling that he believes voters will embrace him as a figure of stability and maturity even in a partisan primary election.
In doing so, Mr. Biden, 76, is making a bet of sorts that the Democratic Party’s leftward shift in recent years has been greatly overstated, and that the moral clarity of his rhetoric and his seeming strength as a general election candidate will overpower other considerations for Democratic voters who tend to prize youth, diversity and unapologetic liberalism.
Laying out for the first time why he wanted to run for president, Mr. Biden invoked the white supremacist march through Charlottesville, Va., that ended in bloodshed in 2017, and Mr. Trump’s comment that there were “very fine people on both sides.” In that moment, Mr. Biden said in the video, “I knew the threat to our nation was unlike any I’d ever seen in my lifetime.”
“We are in the battle for the soul of this nation,” Mr. Biden said, warning that if Mr. Trump is reelected, “He will forever and fundamentally alter the character of this nation, who we are, and I cannot stand by and watch that happen.”
Mr. Trump did not respond to Mr. Biden’s denunciation of his response to Charlottesville, responding instead with a taunt, calling Mr. Biden “Sleepy Joe” and deriding the Democratic field as having “demented ideas.”
Mr. Biden enters the Democratic race as something of a front-runner, albeit one beset by challenges from all flanks and looming questions about his long political record. Allies of Mr. Biden believe he must take steps in short order to explain to voters his political evolution on a range of issues, including ones as elemental as criminal justice, abortion rights and the Iraq war.
In a sign he may recognize the urgency of that task, Mr. Biden recently spoke privately with Anita Hill, who was in 1991 was questioned harshly by a Senate committee led by Mr. Biden after she accused Clarence Thomas, now a Supreme Court justice, of sexual harassment. Mr. Biden expressed regret, according to an aide, but in an interview Ms. Hill said she left the conversation feeling deeply unsatisfied and did not describe Mr. Biden as having apologized.
Mr. Biden’s long-awaited entry effectively completes the list of Democratic candidates, a cast of 20 characters that is the most diverse presidential field ever. Atop it, for now, are two white men in their eighth decades of life — Mr. Biden and Senator Bernie Sanders of Vermont.
Mr. Biden’s position as the leading Democratic candidate is an unfamiliar one for him. His two previous presidential bids, in 1988 and 2008, failed to catch on. Though he campaigned twice as former President Barack Obama’s running mate, Mr. Biden has never been the starring actor in a major political production of his own inception.
The overarching question of Mr. Biden’s campaign is whether he can fill that role with sufficient competence and imagination to both dispel Democratic concerns about his personal discipline and inspire younger voters for whom he is a relatively distant figure.
Mr. Biden is seen by most Democratic voters as a sympathetic figure, a trustee of Mr. Obama’s legacy whose life has been touched repeatedly by grievous tragedy. He has spoken frequently about the death of his first wife, Neilia, and his infant daughter in a 1972 car crash; the death of his son, Beau, in 2015 became an occasion of national mourning.
But Mr. Biden differs in profound ways, in his identity and political orientation, from the rising generation of voters and activists that has increasingly come to define the Democratic Party.
Mr. Biden is a white man who became a senator during Richard Nixon’s presidency, in a party seen as prizing youth and diversity. He is a centrist and a determined champion of bipartisanship, vying to lead a coalition that views the Republican Party as irretrievably malignant. And he plans to finance his campaign chiefly through large contributions from traditional party bankrollers, in an age of grassroots hostility to corporations and the very wealthy.
Mr. Biden has appeared alternately eager to campaign as Mr. Obama’s natural heir, and also wary of subsuming his candidacy entirely in nostalgia for an earlier administration. He did not mention Mr. Obama in his announcement video, and he told reporters, in a brief exchange Thursday at the Wilmington, Del., Amtrak station that bears his name, that he did not want Mr. Obama’s backing at the outset.
“I asked President Obama not to endorse and whoever wins this nomination should win it on their own merits,” Mr. Biden said.
A spokeswoman for Mr. Obama issued a statement on Thursday praising Mr. Biden warmly but not endorsing him, and over the last year Mr. Obama has quietly encouraged a range of other candidates to pursue the presidency.
The dividing line in Democratic politics around Mr. Biden’s candidacy was immediately apparent on Thursday morning. He was instantly endorsed by a number of prominent moderates, including Senators Bob Casey of Pennsylvania and Doug Jones of Alabama, and the International Association of Firefighters is expected to back him early next week. In the crucial early state of New Hampshire, a popular former governor, John Lynch, agreed to help lead Mr. Biden’s campaign.
“I think he’s right that we need to restore the soul of America,” Mr. Lynch said, adding, “I think somebody in the middle has a better chance of beating Donald Trump.”
At the same time, a number vocal liberal activists and advocacy groups offered blunt criticism of Mr. Biden. One of the more influential groups on the insurgent left, Justice Democrats, issued a scathing statement rejecting Mr. Biden as an option in the race and describing him as a symbol of the Democratic establishment that was unable to stop Mr. Trump in 2016.
“The old guard of the Democratic Party failed to stop Trump, and they can’t be counted on to lead the fight against his divide-and-conquer politics today,” said Alexandra Rojas, the group’s executive director.
Mr. Biden is poised to embark on an ambitious and highly visible campaign schedule. After a fund-raising event in Philadelphia on Thursday evening, Mr. Biden is scheduled to appear on Friday on ABC’s “The View,” in his first television interview as a candidate.
Next week, Mr. Biden plans to visit a Pittsburgh union hall on Monday to make remarks on the economy, before embarking on a multi-week tour of the early primary states and California, culminating in a May 18 speech in Philadelphia about “unifying America,” his campaign said.
Facing intensifying scrutiny of his long record, Mr. Biden has yet to allay concerns about the most contentious aspects of his career. In recent months, he expressed remorse — without quite apologizing — for having supported draconian tough-on-crime measures in the 1980s and 1990s, and said he wished the Hill-Thomas hearings before the Senate Judiciary Committee had gone differently.
Kate Bedingfield, a spokeswoman for Mr. Biden, said on Thursday that Mr. Biden had gone somewhat further in a personal conversation with Ms. Hill, voicing “his regret for what she endured and his admiration for everything she has done to change the culture around sexual harassment in this country.”
But Ms. Hill said this week that she could not support Mr. Biden, and told a reporter that he should “give an apology to the other women and to the American public” because of the wide-reaching social impact of the hearings. Mr. Biden is certain to have to address the issue again.
Marc Morial, the president of the National Urban League, said Mr. Biden would bring a unique set of political strengths to the race, but would also need to address aspects of his record that make progressives uneasy.
“I think it’s important that Biden perhaps help people understand that, as a 40-year member of Congress, his views have evolved,” said Mr. Morial, who suggested Mr. Biden might be well-equipped to make an explanation: “He is one of the few guys who is probably as comfortable talking to a group of truck drivers as he is in an African-American church.”
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Mr. Biden’s competitors have already had months to find their footing in the race. The field includes muscular fund-raisers like Mr. Sanders, Senator Kamala Harris of California and former Representative Beto O’Rourke of Texas; intriguing underdogs, like Mayor Pete Buttigieg of South Bend, Ind.; and policy-minded liberals like Senators Elizabeth Warren of Massachusetts and Cory Booker of New Jersey, who have helped frame the race as a contest of ideas.
Mr. Biden’s rivals avoided criticizing him Thursday, and several sent out fund-raising appeals professing confidence in their own prospects. Ms. Harris praised Mr. Biden’s “wealth of knowledge,” before affirming, “I intend to win.” Mr. O’Rourke wrote that the primary was “wide open.”
As the race proceeds, other Democrats take on Mr. Biden’s record more bluntly, including his background as a Delaware senator loyal to the state’s credit card industry and his 2002 vote to authorize war in Iraq.
Mr. Biden’s private endeavors could also become political targets. He has earned millions of dollars through paid speeches and book deals since leaving office, and has created a network of nonprofits and academic centers that employ many of his trusted aides. He intends to shut down the most prominent of those groups, the Biden Foundation.
Rival Democrats have taken encouragement from several unsteady moments in the Biden camp, including his halting response this month to a wave of stories about his physical behavior with women. And his advisers repeatedly explored and then disavowed some offbeat or daring plans, including announcing a running mate early in his campaign.
Mr. Biden’s candidacy is a bet, above all, that none of that will matter in comparison to voters’ alarm at the possibility of Mr. Trump’s re-election.
There are few modern examples of a man of Mr. Biden’s age assuming the leadership of a Western democratic power. The precedents that exist have tended to arise from moments of military conflict or social turbulence: Georges Clemenceau becoming France’s premier during World War I at the age of 76, or Winston Churchill returning as prime minister in the 1950s, also at 76.
Mr. Biden would be 78 on Inauguration Day in 2021, and it remains to be seen whether voters will view him as a similar kind of political savior, or the times as equally dire.