In a video released on Monday morning, Ms. Pelosi drew on her years of working with Mr. Biden behind the scenes on legislation when he was Mr. Obama’s vice president, describing Mr. Biden as “a leader with the humility to seek expertise and science, and the confidence to act upon it.”

“When our nation faced the Great Recession, it was Joe Biden who led the implementation — and the accountability — of the Recovery Act, helping create and save millions of jobs. When the Democratic Congress was passing the Affordable Care Act, Joe Biden was a partner for progress in the White House and also championed the cancer moonshot,” she said.

Ms. Pelosi’s chief focus in the 2020 campaign has been on keeping the House in Democratic hands and helping her members, particularly vulnerable freshman, win re-election in November.

While the primary was still competitive, she had remained carefully neutral, preserving her ability to try to distance her caucus from the eventual nominee if she thought it necessary. But she also had words of praise for Mr. Biden, saying “I’m not counting Joe Biden out” after his early losses in Iowa and New Hampshire.

As many of her rank-and-file members fretted in February about the down-ballot implications of nominating Mr. Sanders, then the front-runner, Ms. Pelosi said she believed Democrats would “wholeheartedly support” whomever the party nominated.