When Joe Biden takes the stage with Barack Obama at an election fundraiser in New York this week, it will be all smiles for a duo who collectively have spent nearly a dozen years in the White House.

But while Biden, 81, will be glad to have the star power of Obama and also former Democratic president Bill Clinton at the event on Thursday, there will also be tensions with his former boss, whom he served as vice president for two terms.

Obama, 62, will be there to support Biden's reelection. But he reportedly expressed concerns about Biden's bid during two lunches in recent months, saying the campaign must do more to prevent a Donald Trump comeback in November.

The White House point blank denies any friction.

"They're close. They're like family. They speak regularly," White House Press Secretary Karine Jean-Pierre said.

But there's no disputing an intriguing dynamic between the cool, cerebral Obama and the older, more folksy Biden who served as vice president to America's first black commander-in-chief from 2009 to 2017.

The ambiguity in their relationship stems partly from the difficulty many vice presidents face in a poorly defined job whose main requirement is arguably just being on hand to replace the president in case the worst happens.

"Presidents and vice presidents from the very beginning have had a fraught relationship," Thomas Whalen, associate professor of social sciences at Boston University, told AFP.

Any strains between Biden and Obama were in any case "small potatoes" compared to some previous mismatched pairings in the long history of US "veeps."

These range from the odd couple of Dwight D. Eisenhower and Richard Nixon to Nixon, on becoming president, thinking his own VP Spiro Agnew was a "complete moron." In the 2000 election, vice president Al Gore froze out Clinton during his tilt at the White House because of the scandal over Clinton's affair with Monica Lewinsky.

By contrast "I think there's a respect there" between Obama and Biden, Whalen said.

'My brother'

The star-studded fundraiser at New York's Radio City Music Hall on Thursday is the first event of its kind to feature the three Democratic presidents.

Biden, Obama and Clinton will be interviewed by late-night TV comic Stephen Colbert and guests can pay $100,000 for a photo with the trio.

For Biden and Obama, it will be a chance to riff on a relationship that saw the odd couple become "such good friends," as Biden put it in a 2017 memoir.

Obama picked Biden because of his long Senate and foreign policy experience. They shared weekly lunches and Biden wrote that Obama "treated me as much as an equal as a president is able to."

Obama famously surprised a tearful Biden with the US presidential medal of freedom in 2017 — saying it "gives the internet one last chance to talk about our bromance."

On a personal level, Obama delivered a moving eulogy for Biden's eldest son Beau, who died of brain cancer in 2015 aged 46, describing his vice president as "my brother."

But there were tensions too.

For years, Biden has smarted over the fact that Obama was "not encouraging" when he was considering a run for the 2016 Democratic nomination eventually won by Hillary Clinton, who despite being heavily favored went on to lose against Trump.

Those hard feelings came up when Biden was interviewed last October by a special counsel investigating whether he had mishandled secret documents.

"Not a mean thing to say. He just thought that she had a better shot of winning the presidency than I did," he said, according to a transcript.

Some in Team Biden now think Team Obama downplays Biden's subsequent achievements — and that they know better when it comes to winning elections.

Reports that Obama late last year urged Biden to speed up the rollout of the campaign and make it less centrally controlled by the White House fueled those tensions.

But those close to Biden say that he welcomes the advice and realizes that he will need the full backing of his Democratic predecessors.

Clinton, 77, who was president from 1993 to 2001, and Biden also frequently speak, they say.

In fact, Obama's concerns were a well-needed "smack on the side of the head," said Whalen.

"Biden is a pro, and I don't think he's going to take it personally. I think some of his staff probably resent it, obviously — but that's to be expected," he said.

Obama also had an eye on history, added Whalen.

"Obama knows the fate politically of Joe Biden is also the historic fate of his administration," he said.