"We are in a battle for the soul of this nation and I'm proud to have the fiercest of fighters, Senator @EWarren, on my side," Biden tweeted in response to her endorsement. "With her help, we're going to beat Donald Trump and create a government that works for everyone — not just the wealthy and well-connected," he wrote, parroting one of Warren's signature lines from the campaign trail.

Warren has mostly stayed quiet about the Democratic primary since her withdrawal on March 5. Some expected her to endorse Sanders, who had been a progressive ally since before she was a senator, but she stayed neutral and diminished Sanders' chances of a comeback. Her silence angered many on the left, including Sanders himself.

Several Warren allies urged her to stay out of the race because they believed Sanders' campaign was effectively over after Super Tuesday and Biden would become the nominee. Her decision to remain neutral may have also boosted her prospects to be Biden's vice presidential pick — her name is one of several often floated in Biden's orbit.

As Biden attempts to bring over many progressives who supported Sanders and Warren, the Massachusetts senator attempted to reassure people on the left who may be skeptical of Biden.

"When you disagree, he’ll listen — not just listen, but really hear you," she said. "And he has 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."

Some campaign staffers echoed her message. "Today we continue our righteous fight with @JoeBiden. The stakes are high, the choice is clear, and we persist," tweeted her video director. "Imagine what our country could be with Joe Biden as commander-in-chief," tweeted her former Nevada state director.

Still, some expressed disappointment with Biden as the nominee. "NGL [not going to lie] it sucks that neither of my preferred candidates became the nominee, but I’m hopeful we can learn from why they both lost and do better next time," Warren's states organizing director wrote Wednesday morning.

Warren also has a complicated history with Biden going back to the early 2000s when Warren opposed bankruptcy reforms that Biden championed as a senator. It got tense: Warren said Biden was trying "to sell out women” and in a hearing with Warren as a witness, Biden said she had made “a very compelling and mildly demagogic argument” in her opposition to the reforms. “I got it, OK. You are very good, professor,” Biden said.

Warren acknowledged those past clashes as she made the case for him Wednesday. "Among all the other candidates I competed with in the Democratic primary, there’s no one who I’ve agreed with 100 percent of the time over the years," she said.

But she also recalled several good moments in their relationship, which people in Warren's orbit say improved over the years even as they continued to have policy differences.

"Joe Biden was there at the very moment I became a senator — he swore me in," Warren said, recalling their interaction in 2013. "And when he did, he said, 'You gave me hell! And you’re gonna do a great job.'"

