Senator Bernie Sanders endorsed Joseph R. Biden Jr. as the Democratic nominee for president on Monday, adding the weight of his left-wing support to Mr. Biden’s candidacy and taking a major step toward bringing unity to the party’s effort to unseat President Trump in November.
In throwing his weight behind his former rival, Mr. Sanders is sending an unmistakable signal that his supporters — who are known for their intense loyalty — should do so as well, at a moment when Mr. Biden still faces deep skepticism from many younger progressives.
The two men appeared via live stream on split screens — each on each other’s live streams — talking to each other. “We need you in the White House,” Mr. Sanders said to Mr. Biden. “And I will do all that I can to make that happen.”
Mr. Biden said: “I’m going to need you. Not just to win the campaign, but to govern.”
Mr. Sanders, who dropped out of the presidential race last week, hinted his intentions in a Twitter post shortly before the appearance. Mr. Biden provided his own clue, saying he would be “joined by a special guest” for his scheduled live stream at 2 p.m.
The scene was a striking example of the ways the coronavirus has upended traditional campaigning. In normal times, both men likely would have appeared onstage together at a rally — or at least done so at an event with more pomp. Instead, both men appeared at their homes, as they have been doing for weeks as they communicate to voters mostly via live streamed events.
At times almost jovial, the two men went back and forth on issues, with Mr. Biden asking Mr. Sanders if he had any questions for him, and Mr. Sanders responding by asking Mr. Biden if he supported policies that the Vermont Senator has championed for years, including a $15 minimum wage and tuition-free public college.
The two men said they would form “task forces” on issues including the economy, education, immigration, health care, criminal justice and climate change.
The scene, which unfolded less than a week after Mr. Sanders ended his own campaign, was a sharp departure from the drawn-out, often-acrimonious process of reconciliation between Mr. Sanders and Hillary Clinton in the 2016 presidential race, and the joint appearance appeared poised to further ease Democratic fears of a divided party headed into a general election against Mr. Trump.
The event followed weeks of discussion between the Biden and Sanders camps over how the two men could find common ground on Mr. Sanders’s key policy priorities. A day after Mr. Sanders left the presidential race, Mr. Biden announced that he was embracing several new, more progressive positions on matters including health care and education, in an explicit overture to Mr. Sanders’s base.
This is a developing story and will be updated.
Astead W. Herndon contributed reporting.