WASHINGTON – As the US election approaches, President Joe Biden is making regular calls to former president Barack Obama to discuss the race or to talk about family.

But Mr Obama is making calls of his own to Mr Jeffrey Zients, the White House chief of staff, and to top Biden campaign aides to strategise and relay advice.

This level of engagement illustrates Mr Obama’s support for Mr Biden, but also what one of his senior aides characterised as Mr Obama’s grave concern that Mr Biden could lose to former president Donald Trump.

The aide, who was not authorised to speak publicly, said that Mr Obama has “always” been worried about a Biden loss. And so, the aide added, he is prepared to “eke it out” alongside his former vice-president in an election that could come down to slim margins in a handful of states.

Perhaps for the first time, the two are on the same page about Mr Biden’s future.

In a sign of things to come, they are to appear together, with former president Bill Clinton, at a major fundraiser for the Biden campaign at Radio City Music Hall in New York on March 28.

It was not always this way.

In 2015, as Mr Biden was mourning the loss of his eldest son, Beau, and contemplating running for the presidency, it was Mr Obama who gently suggested that it was not his time. In a memoir, Promise Me, Dad, Mr Biden wrote that Mr Obama told him that if he “could appoint anyone to be president for the next eight years”, it would have been Mr Biden.

The then Vice-President wrote that “the mere possibility of a presidential campaign, which Beau wanted, gave us purpose and hope – a way to defy the fates”.

But after discussing the stakes with Mr Obama, he took himself out of contention and stepped aside for Mrs Hillary Clinton, seen by the Obama White House as the far stronger candidate.

The decision bred distrust and lasting resentment among some of Mr Biden’s aides. Several of them work in the White House today, and they believe that Mr Obama and his advisers sidelined Mr Biden, whom they think could have changed the course of history and beaten Trump in 2016.

In 2019, when Mr Biden entered the race against then President Trump, Mr Obama withheld his endorsement until after the Democratic primary, although he privately worked to clear a path for Mr Biden.

He also gave his blessing for the Biden campaign to use their interactions in the Obama White House in campaign materials, including footage of when Mr Obama surprised his Vice-President with the Presidential Medal of Freedom shortly before leaving office.

In the 16 years since their first campaign together, the relationship has been defined by its odd-couple characteristics – the Harvard-trained professor and the guy from Scranton, the Senate Foreign Relations Committee chair who went on to serve a former junior member, the cool head and the Irish temper.

Over time, the two gelled, with Mr Biden leveraging his relationships on Capitol Hill to help steer a massive stimulus package through Congress in the Great Recession and to push the Affordable Care Act over the finish line.

He famously congratulated Mr Obama as the President signed the health Bill into law, whispering “Mr. President, this is a big deal” with an inserted adjective not suited for national television.

The two were not aligned on everything.

Mr Biden was vehemently against Mr Obama’s decision to send more American troops to Afghanistan in 2009, a disagreement that would become a focal point for Mr Robert Hur, the special counsel who investigated the current President’s handling of classified documents.

A classified and handwritten document memo that Mr Biden sent to Mr Obama on Afghanistan had been found at Mr Biden’s residence in Delaware by investigators.

Aides to Mr Biden say their relationship morphed from friendly to almost familial after Mr Beau Biden died.

When Mr Obama delivered a eulogy for Beau in June 2015, the then President looked down from the dais and told Mr Biden that he and his family were “honorary members” of the Biden clan.

“And the Biden family rule applies: We’re always here for you, we always will be – my word as a Biden,” Mr Obama said, a moment that has been described by people close to Mr Biden as a major turning point for him, who was stunned by Mr Obama’s publicly affectionate remarks.

But during his interview with Mr Hur, Mr Biden explained another crucial disconnect: Mr Obama’s differing view on Mr Biden’s political future.

Mr Obama and his advisers had chosen Mr Biden for his policy experience, but also because he had what the Obama team thought were limited career prospects beyond the vice-presidency.

Mr Biden recalled to Mr Hur that, while he was pondering a presidential run in 2015, “there were still a lot of people at the time when I got out of the Senate that were encouraging me to run in this period, except the President”, he said, a reference to Mr Obama.

“I’m not – and not a mean thing to say. He just thought that she had a better shot of winning the presidency than I did,” he said, referring to Mrs Clinton.

Still, White House officials and those working for Mr Obama say that any lingering distrust at the staff level has dissipated, given what they see as the urgent need for Mr Biden to beat Trump in November.

Privately, Democrats close to Mr Obama said that their concerns over Mr Biden’s prospects have been assuaged somewhat by the President’s confrontational performance during his State of the Union address.

An e-mail sent by Mr Obama’s alumni group and obtained by The New York Times said as much.

“We hope you are as fired up as we are after the State of the Union!” the group wrote in an e-mail to supporters. “President Biden is ready.” NYTIMES