ATLANTA — President Obama and the former first lady, Michelle Obama, have endorsed Kamala Harris in her White House bid, giving the vice president the expected but still crucial backing of the nation’s two most popular Democrats.

The endorsement, announced Friday morning in a video showing Ms. Harris accepting a joint phone call from the former first couple, comes as Ms. Harris continues to build momentum as the party’s likely nominee after President Biden’s decision to end his reelection bid and endorse his second-in-command against Republican nominee and President Trump.

It also highlights the friendship and potentially historic link between the nation’s first Black president and Ms. Harris, the first woman, first Black woman, and first person of Asian descent to serve as vice president, who is now vying to break those same barriers at the presidential rank.

“We called to say Michelle and I couldn’t be prouder to endorse you and do everything we can to get you through this election and into the Oval Office,” the former president told Ms. Harris, who is shown taking the call as she walks backstage at an event, trailed by a Secret Service agent.

“I can’t have this phone call without saying to my girl, Kamala, I am proud of you,” Mrs. Obama said.

“This is going to be historic,” she added.

Ms. Harris, who has known the Obamas since before his election in 2008, thanked them for their friendship and said she looks forward to “getting there, being on the road” with them in the three-month blitz before Election Day on November 5.

“We’re gonna have some fun with this too, aren’t we?” Ms. Harris said.

The Obamas are perhaps the last major party figures to endorse Ms. Harris formally — a reflection of the former president’s desire to remain, at least publicly, a party elder operating above the fray. The Obamas remain prodigious fundraising draws and popular surrogates at large campaign events for Democratic candidates.

According to an Associated Press survey, Ms. Harris already has secured the public support of a majority of delegates to the Democratic National Convention, which begins August 19 at Chicago.

The Democratic National Committee expects to hold a virtual nominating vote that would, by August 7, make Ms. Harris and a yet-to-be-named running mate the official Democratic ticket.