Sen. Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE (D-Mass.) formally endorsed Joe Biden Joe BidenMomentum growing among Republicans for Supreme Court vote before Election Day Trump expects to nominate woman to replace Ginsburg next week Video of Lindsey Graham arguing against nominating a Supreme Court justice in an election year goes viral MORE on Wednesday, making her the last of the former vice president’s onetime rivals to back him as the Democratic presidential nominee.

The Massachusetts senator and former presidential candidate announced her endorsement in an email to supporters, saying there was “too much at stake” to hold out any longer.

“Joe Biden has spent nearly his entire life in public service,” Warren said. “He knows that a government run with integrity, competence, and heart will save lives and save livelihoods. And we can't afford to let Donald Trump continue to endanger the lives and livelihoods of every American.”

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Warren’s endorsement is the latest sign that the Democratic Party is coming together behind Biden as he prepares to take on President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE in November. It also continues a string of high-profile endorsements for the former vice president.

On Monday, just five days after he ended his own presidential campaign, Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE (I-Vt.) came out in support of Biden during a surprise appearance on one of the former vice president’s campaign livestreams.

And on Tuesday, Biden scored perhaps the most sought after endorsement in Democratic politics when former President Obama backed his former vice president’s White House bid.

With Warren’s endorsement, Biden now has the endorsement of every one of his top former rivals for the Democratic presidential nomination.

Warren sought the party’s nomination for more than a year, running in the same progressive lane as Sanders for much of her campaign. But she failed to gain the kind of traction among young and liberal voters that Sanders had.

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She suspended her campaign last month after a series of disappointing losses on Super Tuesday, including in her home state of Massachusetts.

After her exit from the race, speculation swirled over whether she would endorse Sanders, her chief ideological ally in the race, or Biden, the prohibitive front-runner who appeared on track to beat out Sanders for the nomination.

Warren was sharply critical of Biden throughout her presidential run, berating him at times over his long record as a senator and his corporate ties. Biden, meanwhile, criticized Warren for having what he described as a “my way or the highway” approach to governing.

But after Warren’s exit from the race last month, the former vice president also began making overtures to the Massachusetts senator and her allies, including endorsing her proposal to overhaul parts of a 15-year-old bankruptcy law that the two once clashed over.

Biden still faces deep skepticism from some progressives, who warn that his moderate brand of politics may not be enough to motivate younger and more-liberal voters to go to the polls in November.

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Warren acknowledged in her email to supporters on Wednesday that, throughout her presidential campaign, there was “no one who I’ve agreed with 100% of the time over the years.” But she praised Biden as someone who “will always tell you where he stands.”

“When you disagree, he’ll listen — not just listen, but really hear you. And treat you with respect, no matter where you’re coming from,” she said. “And he has shown throughout this campaign that when you come with new facts or a good argument, he’s not too afraid or too proud to be persuaded.”

Some Democrats have floated Warren as a possible vice presidential pick for Biden, who has already pledged to choose a woman as his running mate. Biden has said that he is considering somewhere between six and 10 candidates for the slot, and it’s unclear if that list includes Warren.

But progressives, in particular, say that tapping the Massachusetts senator for the role could go a long way in courting many liberal voters who remain unsatisfied with having a moderate like Biden as the Democratic nominee.

“At the end of the day, the VP pick is going to make a huge difference,” Norman Solomon, a California-based activist and longtime Sanders ally, said. “There’s only one that has the stature and the capacity to really galvanize progressives to fill Biden’s deficit and that’s Elizabeth Warren.”

Updated at 9:57 a.m.