Obama sometimes struggled to lift other Democratic candidates while he was in the White House, notably losing control of the House in 2010 and the Senate in 2014.
But in the era of President Donald Trump, Democrats believe Obama's appeal, especially among black and younger voters, can help boost energy for Biden.
"There's two groups of voters that Biden needs to move," said Dan Pfeiffer, former White House communications director.
"You have the four million Obama 2012 voters that sat out in '16, Obama obviously has cache with them. And you have to persuade some number of voters who voted for Barack Obama in 2012 and either Trump or a third party candidate in 2016, and Obama obviously is very, very high-performing with those as well."
Obama endorsed Biden with a video message in April, but kept an otherwise low profile throughout the primary and largely avoided wading into national politics.
In recent weeks, however, he's re-emerged publicly to speak out on policing and the civil unrest that followed the police killing of George Floyd in Minneapolis.
Some Democrats say that, in the wake of Floyd's killing, Obama's voice as an advocate for Biden and a leader for the party is needed.
"Biden doesn't have the strongest record on criminal justice reform so having Obama there is helpful in reinforcing that issue," said Ben Tulchin, who polled for progressive Vermont Senator Bernie Sanders' presidential campaign.
"Given what's going on with criminal justice reform and Black Lives Matter, having the first African American president out there publicly backing Biden is extremely helpful."
But Obama's re-emergence is not without risks for Biden.
For Trump's campaign, it offers an opportunity to resurface some of their favourite political attacks — charges that the Obama Administration's policies undermined the American middle class and US interests abroad.
They believe the focus on Obama will help reinvigorate Trump's base, and remind waffling Trump voters — those considering voting for Biden, or staying home — of their dissatisfaction with the prior administration.
And they see a potential opportunity to drive a wedge between Biden and his base by resurfacing issues from the Obama Administration — like the high rate of deportations — that riled progressives during the Democratic primary.
Trump campaign deputy communications director Ali Pardo said that together, Obama and Biden "put 'kids in cages' and failed to stop China from ripping off Americans while overseeing the worst economic recovery since the Great Depression and stagnant wage growth for American workers."
Trump himself has pushed unfounded conspiracy theories about Obama, hoping to taint Biden by association.
Still, Democrats say Obama is eager to take Trump on to defend his legacy in a debate over whose policies have better benefited Americans.
"Trump's election just devastated the country and Obama's legacy," Tulchin said. "Beating Trump is important for his legacy and important for the country."
Biden's embrace of Obama during the Democratic primary created some headaches for the former vice-president within his own party as well.
Biden was criticised by some opponents as too focused on returning to the status quo of the Obama years at a time when the progressive base of the party was clamouring for significant structural change.
But by the end of the primary contest, at least five candidates — including Sanders — aired ads featuring praise from the former president or photos of the candidate alongside him.
And both Biden and Sanders have made overtures towards progressives, with Biden embracing some of Sanders' policies and Obama praising him by name in his endorsement video for Biden.
But Stephanie Cutter, who served as Obama's 2012 campaign manager, said that if Obama's re-emergence into the campaign raises any further debates about the policies of his administration, he'll be prepared to respond.
"There's nobody better to answer those questions than Obama," she said.
- AP
