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In a video posted on Twitter , Biden focused on the 2017 deadly clash between white supremacists and counter protesters in Charlottesville, Virginia. Biden noted Trump's comments that there were some "very fine people" on both sides of the violent encounter, which left one woman dead.
"We are in the battle for the soul of this nation," Biden said. "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."
The 76-year-old Biden becomes an instant front-runner alongside Vermont Sen. Bernie Sanders, who is leading many polls and has proved to be a successful fundraiser . Biden has legislative and international experience that is unmatched in the Democratic field , and he is among the best-known faces in US politics.
He quickly racked up endorsements on Thursday morning, becoming the first Democrat running for president with the backing of more than one US senator.
Still, Biden must compete in a field that now spans at least 20 Democrats and has been celebrated for its racial and gender diversity. As an older white man with occasionally centrist views, Biden has to prove he's not out of step with his party. He's yet to outline his positions on the issues defining the 2020 Democratic primary, most notably "Medicare for All," the universal health care plan authored by Sanders that has been adopted by virtually the entire Democratic field.
His critics in both major political parties were also quick to pounce.
"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," the liberal group Justice Democrats tweeted. "The party needs new leadership with a bold vision capable of energizing voters in the Democratic base who stayed home in 2016."
Biden, a native of Scranton, Pennsylvania, is betting that his working class appeal and ties to Barack Obama's presidency will help him win over such skeptics. Obama has declined to endorse Biden, however, and several former Obama aides are now working for other candidates.
"I asked President Obama not to endorse," Biden told reporters as he arrived in Delaware Thursday. "Whoever wins this nomination should win it on their own merits."
While he hasn't endorsed anyone in the crowded field, Obama took the unusual step of weighing in on Thursday's announcement.
"President Obama has long said that selecting Joe Biden as his running mate in 2008 was one of the best decisions he ever made," Obama spokeswoman Katie Hill said. "He relied on the vice president's knowledge, insight, and judgment throughout both campaigns and the entire presidency. The two forged a special bond over the last 10 years and remain close today."
Trump welcomed Biden to the campaign in a tweet calling him "Sleepy Joe."
"I only hope you have the intelligence, long in doubt, to wage a successful primary campaign," Trump said. "It will be nasty - you will be dealing with people who truly have some very sick & demented ideas. But if you make it, I will see you at the Starting Gate."
Privately, Trump allies have warned that Biden might be the biggest re-election threat given the former vice president's potential appeal among the white working class in the Midwest, the region that gave Trump a path to the presidency.
Biden is paying special attention to his native Pennsylvania, a state that swung to Trump in 2016 after voting for Democratic presidential candidates for decades. While Biden represented Delaware in the Senate for 36 years, he was often referred to as Pennsylvania's third senator.
The former vice president will be in the state three times within the opening weeks of his campaign. He'll be in Philadelphia on Thursday evening headlining a fundraiser at the home of David L. Cohen, executive senior vice president of Comcast. Biden is aiming to raise $500,000 at the event.
He will hold his first public event as a 2020 presidential candidate in Pittsburgh on Monday. Then it's off to Iowa, home of the leadoff nominating caucuses on Tuesday and Wednesday, followed by two days in South Carolina. He'll visit the other two early-voting states, Nevada and New Hampshire, in early May, before holding a major rally in Philadelphia.
Biden's first media appearance is set for Friday morning on ABC's "The View," a move that may help him make an appeal to women whose support will be crucial to winning the primary. He also hired Symone Sanders to serve as a senior strategist, tapping a prominent African American who previously worked for Biden's chief competitor, Bernie Sanders, in the 2016 presidential contest.
As he neared his campaign launch, Biden's challenges have come into greater focus.
He struggled last month to respond to claims that he touched 2014 Nevada lieutenant governor nominee Lucy Flores' shoulders and kissed the back of her head before a fall campaign event. A handful of other women have made similar claims, though none has alleged sexual misconduct.
Biden, a former US senator from Delaware, pledged in an online video to be "much more mindful" of respecting personal space but joked two days later that he "had permission" to hug a male union leader before addressing the group's national conference.
Biden also has been repeatedly forced to explain his 1991 decision, as Senate Judiciary Committee chairman, to allow Anita Hill to face difficult questions from an all-male panel about allegations of sexual harassment against Supreme Court nominee Clarence Thomas, who later was confirmed to the high court.
He has since apologised for his role in the hearing. But in the #MeToo era, particularly after the contentious confirmation of Supreme Court Justice Brett Kavanaugh, the episode remains a significant political liability.
Likewise, Biden once played a key role in anti-crime legislation that had a disproportionately negative impact on African Americans. And while several 2020 Democratic contenders have embraced the possibility of reparations to African Americans for slavery in recent weeks, Biden last month struggled to explain comments he made as a freshman senator in 1975 about the school busing debate.
His first White House bid in 1988 ended after a plagiarism scandal. He dropped out of the 2008 race after earning less than 1% of the vote in the Iowa caucuses. Later that year, Obama named Biden as his running mate.
Boyd Brown, a prominent South Carolina Democrat backing Beto O'Rourke, said Biden's opening salvo stands out.
"This is very strong out of the chute. Well done. Biden just sucked the wind out of the sails for much of the field," he said.
But he noted that announcement bumps fade, and said Biden still has "to campaign the same aggressive way for the next nine months."