Rallying union workers, Biden opens his 2024 campaign. Here’s the latest.

WASHINGTON — President Biden announced his bid for a second four-year term in a video posted online Tuesday morning, urging voters to let him “finish this job” and setting in motion a possible rematch with former President Donald J. Trump.

In the three-minute, four-second video, Mr. Biden said he had spent his first years in office fighting for democracy and freedom. And he warned that “MAGA extremists” around the country — using Mr. Trump’s “Make America Great Again” slogan — threatened those freedoms.

“When I ran for president four years ago, I said we are in a battle for the soul of America. And we still are,” Mr. Biden said in the video, adding later: “That’s why I’m running for re-election.”

The formal declaration of Mr. Biden’s candidacy came four years to the day after he said he would seek the presidency in 2019, asserting at the time that it was vital to prevent Mr. Trump from winning a second term.

Mr. Biden succeeded in 2020. But if Mr. Trump becomes the Republican nominee — he is currently the front-runner — Mr. Biden’s legacy will be determined by whether he is able to once again defeat Mr. Trump.

Here’s what else to know:

• In remarks to union workers hours after making his candidacy official, Mr. Biden received enthusiastic applause as he declared himself “the most pro-union president in American history” and lashed out at Republicans for what he called a “trickle-down” agenda that favored the rich. As he often has, he emphasized his upbringing in Scranton, Pa., telling the crowd that he saw the world “through the eyes of the working people I grew up with” and “through the eyes of people like you.” The speech was an echo of his 2020 announcement, when he spoke at a Pittsburgh union hall after releasing a campaign introduction video online.

• Mr. Biden is unlikely to immediately change his schedule or his message. Aides said he was expected to keep up the same themes he had focused on for months: claiming credit for an improving economy and assailing Republicans on abortion, tax cuts, guns and intolerance. But fund-raising will ramp up quickly: The president’s top donors have been invited to a meeting in Washington on Friday to begin raising money.

• Mr. Biden has all but cleared the Democratic presidential field despite concerns about his age — at 80, he is already the oldest American president in history — and persistent misgivings held by a large number of his party’s voters. Although polls repeatedly show that Democrats yearn for a fresh face in 2024, they just don’t know who that would be.

• Mr. Biden chose Julie Chávez Rodríguez, a top adviser and the highest-ranking Latina in the White House, to be his campaign manager. Quentin Fulks, who managed the 2022 campaign for Senator Raphael G. Warnock of Georgia, will be her deputy. Other members of Mr. Biden’s inner circle — including Jen O’Malley Dillon, his previous campaign manger, and Anita Dunn, his communications guru — will stay in the White House for now.

• The president did not mention Mr. Trump by name in his announcement video. But the subtext was clear — it started with scenes of the Jan. 6 attack on the Capitol. When Mr. Biden said the words “MAGA extremists,” the screen flashed a picture of Mr. Trump with his arm on the shoulder of Gov. Ron DeSantis of Florida, who is expected to seek the Republican nomination. While Mr. Biden’s team is betting that his opponent will most likely be Mr. Trump, the president has already begun ramping up his criticism of all Republicans.