Barack and Michelle Obama have endorsed Kamala Harris in her White House bid, giving the vice president the expected but crucial backing of the nation’s two most popular Democrats.

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

It also highlights the friendship and potentially historic link between the nation’s first black president and 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.

Ms Obama says: “I can’t have this phone call without saying to my girl, Kamala, I am proud of you. This is going to be historic.”

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 going to have some fun with this too, aren’t we?” she said.

The Obamas are perhaps the last major party figures to endorse her 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 the public support of a majority of delegates to the Democratic National Convention, which begins on August 19 in Chicago.

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

Mr Biden endorsed her within an hour of announcing his decision last Sunday to end his campaign amid widespread concern about the 81-year-old president’s ability to defeat Mr Trump.

Former House of Representatives speaker Nancy Pelosi, Senate majority leader Chuck Schumer, House Democratic leader Hakeem Jeffries, House minority whip Jim Clyburn, former president Bill Clinton and former secretary of state Hillary Clinton followed in the days after.

Kamala Harris speaks at the White House complex
Kamala Harris speaks at the White House complex on Thursday (Julia Nikhinson/AP)

The Obamas, however, trod carefully as Ms Harris secured the delegate commitments, made the rounds among core Democratic constituencies and raised more than 120 million dollars (£93 million).

The public caution was in line with how the former president handled the weeks between Mr Biden’s debate debacle against Mr Trump and the president’s eventual decision to end his campaign: Mr Obama was a certain presence in the party’s manoeuvres but he operated quietly.

Mr Obama’s initial statement after Mr Biden’s announcement did not mention Ms Harris. Instead, he spoke generically about coming up with a nominee to succeed Mr Biden: “I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges,” he wrote.

Both Obamas campaigned separately for Hillary Clinton in 2016 and Mr Biden in 2020, including large rallies on the closing weekends before Election Day.

They delivered key speeches at the Democrats’ convention in 2020, a virtual event because of the coronavirus pandemic. The former president’s speech was notable because he unveiled a full-throated attack on Mr Trump as a threat to democracy, an argument that endures as part of Ms Harris’s campaign.