Former US president Barack Obama will endorse the presidential bid of his longtime political ally Kamala Harris in a high-profile speech aimed at boosting her new and untested presidential bid.

At 63, Obama is keen to influence his party behind the scenes and maintains a legacy and a voice that loom large in moments of crisis, including during the messy deliberations that ultimately led President Joe Biden - who also served as Obama’s vice-president - to step out of the race and endorse Harris, 59.

Nearly eight years after the end of his presidency, Obama remains one of the most popular Democrats in the country, eclipsing Harris and the current administration, public opinion polls show.

In a prime-time address to the Democratic National Convention on Tuesday, Obama will lend his own star power to a campaign, light on policy specifics, that has coasted on joyful vibes and the relief of Democrats delighted that Biden’s flailing campaign is over.

Biden took centre stage on Monday, the convention’s opening night, drawing an extended standing ovation from the party faithful and, even with five months left in office, delivering a farewell speech to the party that stretched well past midnight Eastern time.

Like his successors, Obama presided over a closely divided country.

But his victories were enviable by today’s standards for a Democrat: he won in 2008 and 2012 by healthy electoral college margins, carrying not just Michigan, Pennsylvania and Wisconsin, but also Florida, Iowa and Ohio, states now considered out of reach for Harris and running mate Tim Walz.

But his presidency also fuelled the rise of Republican Donald Trump as a titanic right-wing figure, a bitter legacy for liberals that he might reflect upon in his speech.

In his speech, Obama will outline what he believes will lift Harris to victory while also warning Democrats about the tough task they face over the next 11 weeks, according to a source familiar with the remarks.

Harris and Trump are locked in a tight presidential race that will likely be decided in a handful of battleground states, polls show.

“President Obama believes this is an all-hands-on-deck moment, and is committed to doing all he can to elect Vice-President Harris and Democrats across the country,” said Eric Schultz, an Obama adviser.

Obama will make appearances in coming weeks where he believes he can help move the needle and persuade voters, especially in the battleground states likely to decide the election, Schultz said.

In a separate speech, former first lady Michelle Obama will emphasise the need for the country to turn the page on fear and division, according to a source familiar with her plans.

Independent presidential candidate Robert F Kennedy jr is considering ending his campaign to join forces with Trump, Kennedy’s running mate said in an interview posted online on Tuesday.

The vice-presidential candidate, Nicole Shanahan, said that as independents she and Kennedy ran the risk of drawing support from would-be Trump voters and clearing the way for Harris and Walz to win the November election.