WASHINGTON, Wall Street Journal 

Former Vice President Joe Biden said he would seek the Democratic presidential nomination in 2020, launching a third bid for the White House as a blue-collar stalwart in a party hungry to defeat President Trump.

Mr. Biden made his long-awaited announcement Thursday in an online video posted nearly 32 years after he began his first presidential campaign in June 1987.

Mr. Biden announced his candidacy with a three-and-a-half minute online video posted on his social-media accounts. “We are in the battle for the soul of this nation,” Mr. Biden says in the video, pointing to Mr. Trump’s response to events in Charlottesville, Va., the site of a deadly white nationalist rally in 2017. “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.”

He enters a large, diverse primary campaign as an experienced centrist with a reservoir of goodwill among Democrats after eight years as President Obama’s vice president. Early polls have shown him as a leading contender for the nomination, helped by his broad name recognition.

In the months leading up to his announcement, Mr. Biden said he would run as an “Obama-Biden Democrat,” seeking to cast himself as a healing figure for a divided nation and restoring many of the Obama-era policies that have been reversed or challenged by Mr. Trump’s administration.

Mr. Obama isn’t expected to issue an endorsement of Mr. Biden, or any Democratic candidate, during the primaries. Katie Hill, a spokeswoman for Mr. Obama, said that the former president “has long said that selecting Joe Biden as his running mate in 2008 was one of the best decisions he ever made. 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.”

As vice president, Mr. Biden often served as the administration’s emissary to working-class voters in states such as Pennsylvania, Michigan and Wisconsin—states that proved crucial to Mr. Trump’s triumph in the 2016 election.

If he can win the Democratic nomination, Mr. Biden will compete for those blue-collar voters who helped propel Mr. Trump to the presidency.

Mr. Biden is expected to hold his first public campaign event Monday in Pittsburgh, where he plans to address union workers. He is scheduled to attend a private fundraiser in Philadelphia Thursday organized by former Pennsylvania Gov. Ed Rendell and several longtime supporters. A tour of early voting states is expected in the first weeks of Mr. Biden’s campaign.

In the primaries, the 76-year-old former Delaware senator will seek the nomination of a party that has embraced a more expansive government role in health care, education and the environment in recent years.

The former vice president is also expected to face scrutiny of his lengthy record in Congress—including his handling of Anita Hill’s testimony concerning sexual harassment allegations during the 1991 Supreme Court confirmation of Justice Clarence Thomas—as well as his support for the 1994 crime bill that has been criticized by some African-Americans as deepening the impact of mass incarceration on black communities.

More recently, Mr. Biden has faced allegations from seven women that he inappropriately touched them or made close contact. He has said that he would be “more mindful and respectful of people’s personal space.”

Mr. Biden won his first race for Senate in 1972 by a margin of more than 3,000 votes as a 29-year-old—he turned the required age of 30 after the election—and during his six terms in the Senate frequently garnered about 60% of the vote in his races.

The former vice president has acknowledged that he has occasionally committed gaffes that vex members of his own party.

“No one has ever doubted I mean what I say. The problem is that I sometimes say all that I mean,” Mr. Biden quipped during a February appearance at the University of Delaware.

Mr. Biden agonized over a potential presidential candidacy in 2015 in the months after the death of his 46-year-old son, former Delaware Attorney General Beau Biden, of brain cancer, but ultimately decided against a campaign.

His two previous presidential bids ended unsuccessfully.

After announcing his first candidacy in 1987, Mr. Biden dropped out a few months later after admitting he lifted passages of a speech delivered by a British politician.

In the 2008 race, he finished fifth in the Iowa caucuses, receiving less than 1% before folding his campaign.

He joined the Democratic ticket eight months later, serving as Mr. Obama’s sounding board on economic and foreign policy issues during their two terms and traveling to more than 50 countries as vice president.

Mr. Biden was a point person for the president on the conflict in Iraq and led the administration’s efforts to improve the nation’s roads and bridges.

In the spring of 2012, he memorably said during an interview that he was “absolutely comfortable” with gay marriage, irritating White House aides for getting ahead of Mr. Obama’s public support for same-sex marriage.

After Mr. Trump’s election, Mr. Biden devoted himself to the creation of a cancer research institute and played an active role during the 2018 midterm elections, casting Mr. Trump’s presidency as a “battle for the soul of America.”

Mr. Biden has maintained high favorability numbers among Democrats but will need to connect with party activists who will have a broad range of choices in 2020.

The primary field is the party’s largest in a generation, and includes Senate Democrats such as Bernie Sanders of Vermont, Kamala Harris of California and Elizabeth Warren of Massachusetts; newcomers such as South Bend, Ind., Mayor Pete Buttigieg and former Texas Rep. Beto O’Rourke; governors such as former Gov. John Hickenlooper of Colorado and Washington state Gov. Jay Inslee and one of Mr. Biden’s former Obama cabinet colleagues, former U.S. Housing and Urban Development Secretary Julián Castro.

Others are considering a potential campaign, including Montana Gov. Steve Bullock, Colorado Sen. Michael Bennet and New York City Mayor Bill de Blasio.