No candidates offered themselves before the July 27 deadline, ruling out the chance of an open Democratic Party convention in Chicago in August.
Ms Harris appeared to have received the final stamp of approval when the Obamas granted her their endorsement last month.
In a phone call captured for social media, President Barack Obama told her: “We phoned to say Michelle and I couldn’t be more proud to endorse you and do everything we can to get you through this election and into the Oval Office.
The former president joined the Clintons, former House speaker Nancy Pelosi, actor George Clooney, and artists Cardi B, Ariana Grande, and Charli XCX to support Ms Harris.
Donations to the Democrats have also increased since President Joe Biden announced that he would not seek re-election.
Almost immediately, $50 million (£38.7 million) in support flooded in, with labor unions and other groups also backing Ms. Harris.
