Ending months of speculation, former Vice President Joe Biden announced Thursday he is launching his third presidential campaign. Instantly joining the race as an early frontrunner, the former vice president and more than a dozen major Democratic candidates seek to deny President Trump a second term.

"We are in the battle for the soul of this nation," he said in the nearly four-minute long video. "If we give Donald Trump eight years in the White House he will forever and fundamentally alter the character of this nation. And I cannot stand by and watch that happen."

Unlike most of his competitors who only mentioned President Trump in passing when they announced their candidacies, Biden used his video to cast the race as a battle with Mr. Trump for the very soul and character of the country, calling him out by name.

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The video opens in Charlottesville, Virginia, the scene of violent clashes between white nationalists and protesters which opens in Charlottesville, Virginia, where in 2017 there were violent clashes between white nationalists and protesters that resulted in the death of one woman. Mr. Trump, at the time, faced a backlash for saying that blame for the violence fell "on many sides."

Biden announced his bid in a video released on social media. He is expected to hold a fundraiser Thursday evening in Philadelphia and hold his first formal campaign event in Pittsburgh on Monday. In his appeal to voters, Biden recounted the president's response to the violent white supremacist protests in Charlottesville, Virginia two years ago.

Biden said he's running for president because everything about American democracy is "at stake" under a Trump presidency. "We have to remember who we are, this is America," he added in a final appeal to supporters.

Already differentiating himself from the other 17 candidates running, Biden also immediately released Spanish-language advertising to target Latino supporters, which no other candidate did on day one of his or her campaign.

For about a year, many Democratic operatives, donors and voters have been encouraging Biden, who served as vice president for eight years under President Obama, to enter the crowded race, capture the nomination and thwart Mr. Trump's reelection bid in 2020. Despite not having formally announced his bid until Thursday, Biden has been leading several polls among primary voters, with only Vermont Sen. Bernie Sanders coming close to matching the former vice president's support.

People familiar with the campaign's plans said Biden plans to place a particular emphasis on South Carolina, an early primary state where he enjoys strong support from longtime Democratic leaders and the state's large African American community.

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The 76-year-old former Delaware lawmaker, a totem of the Democratic Party's establishment, is likely to quickly attract substantial financial support from the party's more moderate wing, as well as the backing of many top Democrats across the country and in Washington, where he worked for decades.

Unlike many of the other Democratic candidates in the race, Biden is already a household name across the country, and his allies argue he has the best chance to woo working-class voters in states like Michigan, Wisconsin and Pennsylvania who propelled Mr. Trump to victory during the 2016 presidential election.

The former vice president, however, will also face scrutiny from progressives for some of the positions he held during his long tenure in Congress, where he represented Delaware in the Senate for more than 35 years. He has also come under scrutiny in recent weeks by several women who said that he touched them inappropriately at events over the years.

In the 1970s, Biden opposed busing to desegregate public schools. As chairman of the Senate Judiciary Committee, he oversaw the contentious Anita Hill hearings during the confirmation process for then-Supreme Court nominee Clarence Thomas in 1991. He also helped spearhead efforts to pass the Violent Crime Control and Law Enforcement Act of 1994, which many believe fueled a period of mass incarceration that disproportionately affected African Americans and other minority groups.

Biden has since embraced more progressive policy stances. He staged two campaigns for the Democratic nomination in 1988 and 2008, dropping out during the primary contests in both.

The former vice president joins the largest — and the most diverse — Democratic primary field in U.S. history. To date, 20 other Democrats have declared their candidacy for president, including Sens. Sanders, Kirsten Gillibrand, Kamala Harris, Cory Booker and Elizabeth Warren; Texas Rep. Beto O'Rourke; and South Bend, Indiana Mayor Pete Buttigieg.

Ed O'Keefe, Emily Tillett and Caitlin Huey-Burns contributed to this report.