Since Ms. Warren ended her own campaign, she and Mr. Biden have spoken multiple times about policy issues, including Mr. Biden’s plan to respond to the coronavirus pandemic. At the staff level, Ms. Warren’s chief campaign strategist, Joe Rospars, and Anita Dunn, a senior adviser to Mr. Biden, have been engaged in talks bridging the two camps.

In late March, Mr. Biden endorsed forgiving at least $10,000 in federal student loans and credited Ms. Warren for the proposal. Over the weekend, he said during his podcast that he supported immediately increasing Social Security payments by $200 a month, another proposal that Ms. Warren had championed.

“One thing I appreciate about Joe Biden is that he will always tell you where he stands,” Ms. Warren said in her endorsement video. “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. 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.”

The Biden campaign has broadly signaled a willingness to hire staff from formal rivals as they gear up for the general election matchup with Mr. Trump, and some outreach is already underway, but there have been no promises to hire a specific number of aides, a person familiar with the matter said.

While the daily stream of endorsements this week was unmistakably designed to signal party unity, Mr. Biden has also sometimes struggled to drive a consistent message of his own amid the coronavirus outbreak. He has often been overshadowed by Mr. Trump and the governors who are on the front lines of the fight. This week, he was back in the spotlight.

“It’s a difficult news environment,” said Jennifer Palmieri, a former senior aide to Hillary Clinton and Mr. Obama. “I think it’s smart to do a dramatic show of force and have everyone go in one week, because it just serves to raise the stakes for Democrats. It raises the stakes for November.”

“You sacrifice the longevity of the story line,” Ms. Palmieri added, “but I think you want to have the drama of everyone going in one week rather than the drumbeat.”