President Joe Biden led the charge, endorsing Kamala Harris within minutes.
Bill and Hillary Clinton were quick to follow, pledging their support for the vice president.
Other names once touted as potential substitutes for Mr Biden rallied behind – Governors Gavin Newsom and Josh Shapiro.
But there was one name conspicuous by its absence on the cheerleading list – President Barack Obama.
‘Honoured’ Harris gets top Democrats’ backing; follow US latest
A statement from the Obamas’ office lauded Mr Biden but made no reference to Ms Harris.
“We will be navigating unchartered waters in the days ahead,” it read.
“But I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges.
“I believe that Joe Biden’s vision of a generous, prosperous and united America that provides opportunity for everyone will be on full display at the Democratic Convention in August.”
That could be interpreted as a call for an open contest, rather than a coronation.
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Would the man who made history as America’s first black president not want his party to break new ground again?
If she won the nomination and subsequent election, Ms Harris would be the first woman to take the White House.
President Joe Biden and former President Barack Obama in March. Pic: AP
A source close to Barack Obama said: “Just like he did in 2020, once Joe Biden earned the nomination, President Obama believes he will be uniquely positioned to help unite the party.”
Mr Obama did not endorse Mr Biden in 2020 until Senator Bernie Sanders had dropped out of the race.
Speculation that he is holding back in the hope of his wife Michelle winning the nomination appears without foundation.
While polls suggest she is the one Democrat who could beat Donald Trump, the former first lady has repeatedly insisted she has no presidential aspirations.