Former Vice President Joseph R. Biden Jr. announced Thursday that he would seek the Democratic nomination to challenge President Trump in 2020, marshaling his experience and global stature in a bid to lead a party increasingly defined by a younger generation that might be skeptical of his age and ideological moderation.
Mr. Biden, 76, is set to offer himself as a levelheaded leader for a country wracked by political conflict, a rationale he believes could attract a broad cross-section of voters who want to move on from Mr. Trump. Mr. Biden is seen by many Democrats as a trustee of former President Barack Obama’s legacy, perhaps capable of restoring the consensus-seeking liberalism of Mr. Obama’s administration.
The former vice president has encouraged that perception, labeling himself to reporters in early April as an “Obama-Biden Democrat” and suggesting that accounts of the left wing’s ascendancy in the party were greatly exaggerated.
“We are in the battle for the soul of this nation,” Mr. Biden said in a three-and-a-minute video laying out his reasons for running. “I believe history will look back on four years of this president and all he embraces as an aberrant moment in time. But if we give Donald Trump eight years in the White House, he will forever and fundamentally alter the character of this nation, who we are, and I cannot stand by and watch that happen.”
As he joins a race with 19 other Democratic candidates, Mr. Biden is in an unaccustomed political position: He is an early front-runner for the nomination, though by no means an imposing one. He has run four previous national campaigns — two as a little-noticed candidate for the presidential nomination in 1988 and 2008, and two as Mr. Obama’s running mate — but in a half-century career he 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, and dispel concerns within his party that he lacks the discipline to run an effective campaign or the vision to ignite Democrats’ enthusiasm.
His long-awaited entry effectively completes the field of major Democratic candidates, and may goad the party’s large number of would-be presidents to compete more aggressively for attention in a race currently framed by two outsize political characters in their eighth decades of life — Mr. Biden and Senator Bernie Sanders of Vermont.
Mr. Biden’s rivals have taken encouragement in recent weeks from signs of unsteadiness and indecision in his camp. Mr. Biden spent much of this month attempting, in fits and starts, to address a wave of stories in which women described his physical manner as discomfiting and excessively intimate. And his advisers have repeatedly explored and then disavowed a range of offbeat or daring plans, including announcing a running mate early in his campaign — perhaps Stacey Abrams, the Georgia Democrat — or kicking off his campaign in Charlottesville, Va., as a rebuke to Mr. Trump for his handling of a 2017 white supremacist rally there.
It is unclear how bold a campaign Mr. Biden intends to run, and whether he will seek to electrify the Democratic coalition or merely satisfy its thirst for a champion who appears up to the job of beating Mr. Trump.
Mr. Biden is also expected to face intensive scrutiny of his decades-long political record as vice president and a Delaware senator, and his political allies believe he must take steps in short order to articulate publicly how his views have changed over time on fundamental Democratic concerns involving race and women’s rights. He has yet to allay concerns about the most contentious aspects of his career, including his treatment of Anita Hill during the confirmation hearings of Justice Clarence Thomas.
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 elements of his record that progressives might view as suspect. Mr. Morial, who is not backing a candidate, said he had spoken with Mr. Biden this week and urged him to attend the National Urban League’s July conference to lay out his views on civil rights.
“I think it’s important that Biden perhaps help people understand that, as a 40-year member of Congress, his views have evolved,” Mr. Morial said, suggesting that 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.”
Mr. Biden has consistently led the Democratic primary polls, collecting between a quarter and a third of the vote, and he is expected to receive robust support from donors in the Democratic establishment and the national business community. Last week he began accepting financial contributions for his candidacy, and his wealthy supporters are expected to hold a major fund-raising event in Philadelphia to help propel him into the race.
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Mr. Biden deliberated for months over whether to embark on another run for the presidency, his third over four decades. His two previous campaigns fizzled: in 1988, after allegations of rhetorical plagiarism, and in 2008, when he found himself unable to compete with the star power of Mr. Obama and Hillary Clinton.
He agonized over whether to oppose Mrs. Clinton for their party’s nomination in 2016, but ultimately decided against running after the death of his son, Beau, from cancer.
This time, Mr. Biden has described himself to associates as the candidate best equipped to defeat Mr. Trump, mainly by reclaiming historically Democratic areas of the Midwest, where many lower-income white voters have abandoned his party in a broad cultural and racial realignment. He is said to view the election as the equivalent of a national emergency, with no task more urgent than denying Mr. Trump a second term.
The Democratic base may or may not share Mr. Biden’s assessment of his own strength. Some Democrats’ reservations about Mr. Biden hardened in recent weeks as he offered a halting response to women who came forward to say they had been uncomfortable with his physical mannerisms in interactions stretching back decades.
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A white male centrist in his 70s, Mr. Biden also resembles few of the Democrats who electrified the party during last year’s midterm elections. And because he delayed so long in joining the race, Mr. Biden now finds himself up against an array of competitors who have already found their footing in different ways.
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., who have excited voters with their novelty; 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.
Some of those candidates may be keen to take on Mr. Biden’s record, including his background as a Delaware senator who was highly attentive to the state’s credit card industry. Ms. Warren has been bluntly critical of Mr. Biden in the past, and much of his legislative history conflicts with the central concerns of Mr. Sanders, the leading liberal populist in the race.
Mr. Biden has already been facing tough scrutiny of his shifting policy positions over the decades, on matters like civil rights, abortion and the war in Iraq.
Confronting skepticism from his own party, Mr. Biden has recently expressed regret for supporting stringent tough-on-crime measures in the 1980s and 1990s, and he made a partial attempt at voicing contrition for the aggressive questioning Ms. Hill faced when she accused Justice Thomas of sexual harassment.
Mr. Biden’s private endeavors and personal finances could also become political targets. He appears to have earned upward of $12 million since leaving office, mainly through a book deal and dozens of paid speeches, and he has created a network of nonprofits and academic centers that employ his advisers and advocate his priorities. His aides have drawn up plans to shut down his flagship nonprofit, the Biden Foundation, now that he is a candidate.
Mr. Biden promised in January, through a spokesman, that he would give a full accounting of his assets if he entered the presidential race, including his tax returns.
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.
Mr. Biden’s imposing tenure in government has long been twined in voters’ eyes with a biography touched several times by crippling tragedy. He has spoken frequently about the death of his first wife, Neilia, and his infant daughter in a 1972 car crash. And Beau Biden’s death in 2015 became an occasion of national mourning.
In matters beyond grief, too, Mr. Biden is widely seen as a politician with an uncommon gift for empathy, and for communicating with voters on an emotional level. He has criticized Democrats for appearing too “elitist,” and during the midterm elections last year branded himself happily and often as “Middle-Class Joe.”