The Massachusetts Democrat, a one-time presidential primary rival of the former vice president, made the announcement online. In a video, Warren highlighted Biden's work during the recovery from the 2008 financial crisis and his efforts to soothe her state's largest city after the 2013 Boston Marathon bombing — casting him as a crisis manager as the coronavirus pandemic ravages the country.
"He knows that a government run with integrity, competence, and heart will save lives and save livelihoods," Warren said in the more than three-minute video. "And we can't afford to let Donald Trump continue to endanger the lives and livelihoods of every American. And that's why I'm proud to endorse Joe Biden as president of the United States."
Her support for Biden, the apparent Democratic presidential nominee, comes a day after former President Barack Obama endorsed him. Sen. Bernie Sanders, a Vermont independent and Biden's final primary opponent, supported Biden on Monday only a few days after he dropped out of the race.
In a statement responding to Warren's endorsement, Biden said he had "no competitor more passionate in her convictions or sharper in her arguments" than the senator. He added that the detailed policy plans for which Warren has become known are "more important than ever" as the pandemic wrecks the U.S. economy.
"We know how much work it will take to come through this crisis, and I am proud to have Senator Warren in my corner for the fight ahead — not just as we work to defeat Donald Trump in November, but in the years to come, as we push through a bold and progressive policy agenda for the American people," he said.
With Warren's support, all of the serious contenders for the 2020 Democratic nomination have backed Biden. Her Senate colleagues — including those in presidential battlegrounds — have also started to coalesce around the former vice president. Sens. Tammy Baldwin, D-Wisc., and Debbie Stabenow and Gary Peters, D-Mich., backed Biden on Wednesday, following Sen. Sherrod Brown, D-Ohio, earlier this month.
The senator, who built her campaign around a steep wealth tax, breaking up large companies and transitioning to "Medicare for All," has views that at times clash with Biden's. When Biden first entered the presidential race, Warren — a fierce advocate for consumer protection and stricter bank regulations — criticized him over his support in the Senate for a 2005 bankruptcy bill that she opposed.
Since Warren left the race last month, Biden has tried to win over her supporters by adopting some of her policies. He backed her plan to change bankruptcy rules, including by allowing people to shed student loans during a bankruptcy process in a similar way to other debts.
He also called to forgive $10,000 per person in federal student loan debt during the coronavirus outbreak, a plan put forward by Warren and other Democratic senators.
In the endorsement video, Warren acknowledged her differences with Biden — a man she labeled a "Washington insider" during the campaign. She said she had no Democratic primary opponents "I've agreed with 100% of the time over the years." But she added that Biden will "always tell you where he stands" and "listen."
"And he's shown throughout this campaign that when you come up with new facts or a good argument, he's not too afraid or too proud to be persuaded," Warren said.
Biden is the last Democrat standing in the presidential primary, but it is unclear when he will become the nominee. He has amassed 1,288 pledged delegates — about two-thirds of what he needs to become his party's standard bearer, according to NBC News.
But since a slew of states pushed their primaries back to June, he will not hit the delegate threshold for weeks.
Since she left the presidential race, Warren has proposed a string of coronavirus relief measures in the Senate. Along with the debt forgiveness proposal, she has put forward a plan calling for higher pay, guaranteed sick leave and stronger safety protections for people who have to go into their workplaces during the pandemic.
On Wednesday, she also proposed a $50 billion injection to stabilize the child care system during the pandemic, to ensure parents are able to go back to work.