Former U.S. president Barack Obama and his wife 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 Harris accepting a joint phone call from the former first couple, comes as Harris 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 Harris, who is shown taking the call as she walks backstage at an event, trailed by a Secret Service agent.

Said Michelle Obama, "I can't have this phone call without saying to my girl, Kamala, I am proud of you.

"This is going to be historic," she added.

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 Nov. 5.

"We're gonna have some fun with this too, aren't we?" Harris said.

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Widespread support in less than a week

The Obamas are perhaps the last major party figures to endorse 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, Harris already has secured the public support of a majority of delegates to the Democratic National Convention, which begins Aug. 19 in Chicago. The Democratic National Committee expects to hold a virtual nominating vote that would, by Aug. 7, make Harris and a yet-to-be-named running mate the official Democratic ticket.

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Biden endorsed Harris 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 Trump. Former House 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.

The Obamas, however, trod carefully as Harris secured the delegate commitments, made the rounds among core Democratic constituencies and raised more than $120 million US. The public caution tracks how the former president handled the weeks between Biden's debate debacle against Trump and the president's eventual decision to end his campaign: Obama was a certain presence in the party's manoeuvres but he operated quietly.

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

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