Sen. Bernard Sanders on Monday endorsed Joseph R. Biden’s presidential bid, doing his part to unify the Democratic Party and signaling to his die-hard supporters that rallying behind the former vice president is the best way to stop President Trump and advance their liberal vision.
Less than a week after suspending his campaign and seven months from Election Day, Mr. Sanders appeared beside Mr. Biden on a livestream split screen to announce he was all-in for the presumed nominee.
“Today, I am asking all Americans, I’m asking every Democrat, I’m asking every independent, I’m asking a lot of Republicans to come together in this campaign to support your candidacy, which I endorse, to make certain that we defeat somebody who I believe, and I am speaking just for myself now, is the most dangerous president in the modern history of this country,” Mr. Sanders said.
“We have to make Trump a one-term president, and we need you in the White House,” he said. “So I will do all that I can to see that that happens, Joe.”
Mr. Biden, who appeared at times to be reading from cue cards, thanked Mr. Sanders. “I’m going to need you not just to win the campaign, but to govern,” he said.
Mr. Biden is hoping the endorsement will help him make inroads with the young and far-left voters who flocked behind Mr. Sanders’ bid and view the former vice president as part of the Democratic “establishment” that they rage against.
It is a vastly different scenario from the one in 2016, when Mr. Sanders slow-walked his endorsement of Hillary Clinton and was later partly blamed for her stunning loss.
Since then, Mr. Sanders has driven the policy discussion in the party and won concessions from party officials by reducing the power of superdelegates at the Democratic National Convention.
Although many Democrats at the exit polls said they supported Mr. Sanders’ policy ideas, they overwhelmingly voted for Mr. Biden, including in Wisconsin, whose results from last Tuesday’s primary were announced Monday.
The Biden and Sanders camps have been intent on avoiding a repeat of the nasty fight four years ago, and the two men said Monday that they have been working on “task forces” on issues of importance to Democrats and liberals.
Mr. Biden also has made overtures to Sanders supporters.
Last week, he announced his support for lowering the Medicare eligibility age to 60 from 65 and forgiving loans for students who attended public colleges and universities and private historically black colleges and universities.
But some Sanders supporters are looking for more. Among them is Rep. Alexandria Ocasio-Cortez, the New York firebrand who served as a top surrogate for Mr. Sanders and who has said she and Mr. Biden would be in different political parties in any other nation.
In an interview published Monday in The New York Times, Ms. Ocasio-Cortez said the Medicare proposal from Mr. Biden is a warmed-over idea masquerading as something new.
“And it’s almost insulting,” she said. “I think Hillary was looking at policies that lowered it to 50. So we’re talking about a “progressive concession” that is 10 years worse than what the nominee had in 2016,” she said.
Eight grassroots groups, including Justice Democrats, NextGenAmerica and the Sunrise Movement, have urged Mr. Biden to adopt policy proposals that are popular with young liberals, including a wealth tax, an abolishment of the Senate filibuster and a homeland security secretary committed to “dismantling” Immigration and Customs Enforcement and Customs and Border Protection.
Briahna Joy Gray, a former national press secretary for Sanders 2020, said she isn’t ready to back Mr. Biden.
“I supported Bernie Sanders because he backed ideas like #MedicareForAll, canceling ALL student debt, & a wealth tax,” she said in a Twitter post. “Biden supports none of these.”
“I look forward to seeing what these task forces come up with: The support of a multitude of progressives hinges on Biden making meaningful concessions to the left — not just lip service — not merely symbolic overtures,” she said.
Others said they were surprised by the speed of the endorsement, and some speculated that the Biden camp could have tipped off Mr. Sanders that former President Barack Obama was getting ready to endorse and wanted to give Mr. Sanders a chance to shine.
“His support was going to happen, and while it happened a little bit earlier and in a more surprising fashion than folks expected, this was going to happen, and it reflects at the same time that even the leader of a movement is just one voice in that movement,” said Neil Sroka, a spokesman for Democracy for America.
“Vice President Biden is going to have to continue to earn the support of Sen. Sanders supporters over the weeks and months ahead,” he said.
David Axelrod, a former senior adviser to Mr. Obama, applauded Mr. Sanders.
“This quick, warm and unambiguous endorsement from @BernieSanders is a huge and important boost for @JoeBiden,” Mr. Axelrod said on Twitter. “Without papering over their differences, the two projected a united front. Each handled this well.”
Mr. Biden said Mr. Sanders deserves more credit for “being a voice that forces us all to take a hard look in the mirror and ask if we’ve done enough.”
“He has put the interest of the nation — and the need to defeat Donald Trump — above all else,” Mr. Biden said in a follow-up email to supporters.
“And to Bernie’s supporters, I make the same commitment: I see you, I hear you, and I understand the urgency of what it is we have to get done in this country,” he said.
Brad Parscale, Trump 2020 campaign manager, said the Sanders endorsement underscores how far to the left Mr. Biden has moved.
“One thing that is missing is enthusiasm, however, as almost no one is excited about a Biden candidacy,” he said. “And while Biden is the Democrat establishment’s candidate, President Trump remains the disruptor candidate who has brought change to Washington.
“President Trump’s supporters will run through a brick wall to vote for him,” he said. “Nobody is running through a brick wall for Joe Biden.”