Gabriel Debenedetti is the national correspondent for New York magazine. His first book brings depth and context to the near-two-decade relationship between the 44th and 46th presidents. Under a telling subtitle, The Imperfect Union of Joe Biden and Barack Obama, Debenedetti captures the two men’s closeness – and distance.

The Long Alliance emphasizes that the pair’s time in power together was not a buddy movie. Obama was the star. Biden played a supporting role until he too seized the brass ring, to send Donald Trump into exile.

Obama was a first-term senator, just 47 years old, when he vanquished the Clintons, bulldozed John McCain and entered the White House. Biden’s trajectory was markedly different. Late in life, on his third quest for the presidency, he took down another septuagenarian amid a deadly pandemic.

The union of Obama and Biden was always moored in intergenerational convenience. Obama was the agent of change, Biden a relic of an older time. Obama’s aides cast a wary eye toward the senator from Delaware. To Biden, politics was tactile. He did not readily inspire.

Walloped by Obama in Iowa in 2008, Biden immediately withdrew. Over time, the two men bonded. There was greater warmth between them than between Ronald Reagan and George HW Bush, let alone Bush and Dan Quayle. Obama always heard Biden out. On the other hand, the Obamas never invited the Bidens to the White House residence. Barack and Joe shared lunches, not dinners and movies with popcorn.

Hiccups and speed bumps left marks. Biden got ahead of Obama on gay marriage. Hunter Biden made headlines with his schemes and hustles. Confronted with the younger Biden boy’s foray into Ukraine and the energy business, Obama’s spokesman, Jay Carney, expressed discomfort. Like Trump, Hunter’s fate now rests with federal prosecutors.

Obama empathized with his vice-president. When Beau Biden, Biden’s older son, was dying, Obama offered a shoulder to lean on. He delivered a stirring eulogy. In their final days in office, Obama gave Biden the presidential medal of freedom. The honor, suffused with affection and tenderness, surprised its recipient. Biden’s successor as vice-president, Mike Pence, met a very different fate.

Yet for all Obama’s smarts, he could get things terribly wrong. He failed to anticipate the magnitude of the backlash to the Affordable Care Act, the resonance of birtherism, abhorrent as it was, and the depth and breadth of the emerging national chasm beneath him. Democratic losses in the 2010 and 2014 midterms and the Tea Party with its tricorn hats presaged a sustained demand for a return to the past, the rise of Trump and a tolerance for autocracy within the Republican party.

Obama also messed up by viewing Hillary Clinton as his rightful successor, if not his political heir. In 2008, competing against her for the nomination, he derided her as “likable enough”. In 2016, in hindsight, little had changed.

Clinton lacked her husband’s capacity to emote and connect. Like Ted Cruz, the Republican Texas senator, there was something awkward, off-putting, which she could not shake. Her comments on Trump’s “deplorables” hurt her much as Mitt Romney’s take on the “47%” did him in 2012. Looking back, Obama miscalculated – much as his brain trust would do in 2020 with Biden.

Under Trump, Romney showed a deeper appreciation of where the US stood. It wanted a president not named Trump. A shot at normalcy. Nothing else.

On the night of the 2018 midterms, Romney urged Biden to wage one more campaign. “You have to run,” Romney said in a call. Anti-Trump sentiment cost the Republicans the House but at the same moment Utah was sending Romney to the Senate.

During the 2020 primaries, Obama and Biden stayed in touch. But until the former vice-president emerged as the presumptive nominee, his president’s endorsement was not forthcoming. Biden lost Iowa, New Hampshire, Nevada. Heading to South Carolina, he was low on cash and short on delegates. There, the backing of James Clyburn, the House whip, together with the state’s Black voters, righted Biden’s ship. Debenedetti shows mastery of the tugs and crosscurrents that shape the Democrats’ upstairs-downstairs coalition.

African Americans could be among the most socially conservative components of the party. They were not clamoring for open borders or Medicare for All. Obamacare stood as the legacy of the first Black president. Their patrimony was the cruel lash of slavery, not the Harvard faculty lounge or the yoke of the tsar. Elizabeth Warren and Bernie Sanders did not speak to them or for them.

Obama aides badmouthed Biden in print and on TV. David Axelrod, a senior Obama campaign and White House hand, never cottoned to Biden, and Biden knew it. And yet, behind the scenes, Obama helped clear the field.

In the end, Covid and the need for national leadership put Biden over the top. No other Democrat could have beaten Trump.

As president, Biden’s record is uneven. The withdrawal from Afghanistan put a dent in his standing from which he has not recovered. In contrast, US support for Ukraine appears the product of thoughtful conviction. As for the economy, Biden’s efforts to placate his base may well have heightened inflation. Gas prices are coming down but the rest remains stubbornly up.

Biden competes with Obama’s legacy and the ghost of FDR. The Democrats hold only 50 Senate seats, control on a knife-edge as the November election looms.

“I am confident that Barack is not happy with the coverage of this administration as more transformative than his,” Biden reportedly told one adviser, according to another big political book, This Will Not Pass by Jonathan Martin and Alex Burns, of the New York Times and CNN respectively.

The two men still talk, though.

• The Long Alliance: The Imperfect Union of Joe Biden and Barack Obama is published in the US by Macmillan