Former U.S. President Barack Obama will endorse his longtime political ally Kamala Harris on Tuesday night in a high-profile speech that aims to boost her presidential bid over Republican Donald Trump.

At 63, Obama maintains a legacy that looms large in moments of crisis, including during the messy deliberations that ultimately led President Joe Biden to step out of the race and endorse Harris, 59.

A video of Obama and his wife Michelle Obama congratulating Harris in a phone call has been viewed by millions, and top advisers from Obama's 2008 presidential bid have signed on with Harris' campaign. The former president now reaches out to Harris with advice, according to a Harris aide.

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 in Chicago on Tuesday, the first Black U.S. president lends his own star power to a campaign, light on policy specifics, that has to date coasted on joyful vibes and the relief of Democrats delighted that Biden's flailing campaign is over.

Biden took center 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 (0400 GMT).

Like his successors, Obama presided over a closely divided country. But his victories were enviable by today's standards: he won in 2008 and 2012 by healthy Electoral College margins, carrying states like Florida, Iowa and Ohio that are now considered out of reach for Harris and running mate Tim Walz.

But Trump also launched his political career through racist attacks on Obama's citizenship status and then succeeded him in the White House, a bitter legacy for liberals that Obama may 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.

"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 persuade voters, especially in the battleground states likely to decide the election, Schultz said.