An explosive new biography on President Biden is laying bare an administration led by an irritable elderly man who flip-flops on issues when he isn’t making crass comments about former President Barack Obama.

Biden, 80, was revealed to be hell-bent on distinguishing himself from Obama, his former boss and predecessor, whom he considered measured to a fault, according to “The Last Politician’’ by Franklin Foer released Tuesday.

The author — who says he had “unparalleled access to the tight inner circle of advisers who have surrounded Biden for decades” — describes Biden as mercurial and quick to rush to judgment, even against his top aides’ and experts’ advice.

Among the book’s claims:

The president was ready to renege on his 2020 campaign promises to open the US southern border to thousands more migrants during furious meltdowns just two months into his administration.

From the start of his presidency, Biden’s “angry moods made for uncomfortable meetings” about the border crisis, Foer writes.

On the campaign trail, Biden had promised “to commit himself to a wholesale reversal of Trump immigration policy” to win the election. But privately he “hated to see how his campaign proposals were being translated into policy.”

For example, Trump set a historic cap to allow just 15,000 refugees into the country per year, and Biden — despite his campaign claims — was hesitant to change it.

“Biden hadn’t realized that reforms would go so far,” Foer writes. “When he learned that [US immigration agents] might stop targeting fentanyl dealers, sex offenders and other felons, he exploded in anger.”

On Feb. 12, his administration told Congress that Biden would not cancel Trump’s cap and instead would raise it from 15,000 to 62,500 refugees.

Now all Biden had to do was approve the funding for it, “but in his cantankerous mood, he doubted whether he should,’’ Foer says.

“They want me to increase the number of people in the country, but that’s kind of crazy,” Biden told Secretary of State Antony Blinken, according to the book.

Blinken, “a passionate proponent of raising the cap,” objected to Biden’s waffling, enraging the president so much that “his obstinacy [to raising immigration caps] deterred aides who might have otherwise challenged him, for fear of wasting precious capital on a hopeless fight,” Foer writes.

On April 16, 2021, the White House announced that Biden would keep Trump’s refugee cap through the remainder of the fiscal year, but “the cries of betrayal were instantaneous” from the left, Foer says.
Biden kept “raising the subject himself . . . usually with an edge of aggression.

“He moaned, ‘Can you believe that they want me to go back to those high numbers?’ ” writes Foer, staff writer for The Atlantic and former editor of The New Republic.

After months of wrestling with his progressive advisers, Biden ultimately agreed to fund the annual resettlement of up to 125,000 migrants in the US.

Despite his status as the nation’s oldest president in history, Biden feels he is “babied” by staff — often setting off his infamous Irish temper.

The gaffe-prone president’s staff regularly scrambles to mop up the messes Biden makes with off-script comments, but the efforts are not always appreciated, Foer says.

That tension came to a head in the early days of Russia’s war on Ukraine when Biden, through excitement, bombed what otherwise would have been a triumphant speech supporting Kyiv.

Instead of ending his speech as planned, Biden culminated it with an off-the-cuff statement that sounded as if he was calling for Russian President Vladimir Putin to be overthrown: “For God’s sake, this man cannot remain in power.”

The statement alarmed Americans and international allies who worried it would spark retaliation and escalation from Putin and his arsenal of nuclear weapons.

Upon leaving the stage, “Biden instantly knew that the White House would have to clarify his mistake,” according to Foer.

No stranger to walking back comments made by the president, Biden’s staff quickly explained away the controversial statement — but without consulting him.

That prompted the mercurial president’s ire and resentment.

“By the time [he had] piled into the motorcade leaving [the speech], his aides had released a statement walking back his sentence,” Foer writes.

“Suddenly, the press wasn’t marveling at his rhetoric or his diplomatic triumphs; it was back to describing him as a blowhard lacking in self-control.”

The coverage enraged Biden — who Foer says, “like Donald Trump . . . is a voracious consumer of television” — and he “left for home, ending his triumphalist tour, feeling sorry for himself.”

Deep down, he “knew that he had erred” by making the comment in the first place. But instead of taking responsibility for the flub, he “resented his aides for creating the impression that they had cleaned up his mess.”

“Rather than owning his failure, he fumed to his friends about how he was treated like a toddler,” Foer writes. “Was John Kennedy ever babied like that?”

Biden liked to joke during the height of the pandemic that controversial ex-COVID czar Anthony Fauci could replace Kamala Harris as his No. 2.

As Biden’s team scrambled to tackle the coronavirus in 2021, the president would quip during meetings that his then-White House chief medical adviser should sit in the vice president’s chair.

“When the president assembled his COVID team, he would jokingly direct Anthony Fauci to sit in the vice president’s chair,” Foer writes.

For his part, Fauci “struggled to contain his exuberance” about working with Biden.

A new administration was in charge of ending the pandemic — one that would listen to him, unlike Trump’s.

Fauci had been “traumatized” by public-speaking events with Trump, who had infamously asked scientists if there was a way to inject disinfectant into humans to kill the coronavirus. Trump later claimed the comment was “sarcastic.”

“When Fauci made his way to the podium in the White House briefing room for the first time after the [Biden] inauguration, he felt a surge of traumatic memories wash over him,” Foer writes. “For the first time in years, he was no longer in the awkward position of having to correct the administration he served.”

But Biden would harangue Fauci during his first month as president with a “challenging question,” according to the book: “When would the nation return to normal?”

The president was deeply reliant on the controversial figure’s advice and assistance, so much so that when other public health officials attempted to draft an answer, “Biden would disregard them and ask to speak with Fauci,” the author says.

As vice president, Biden griped that then-President Obama couldn’t even properly say, “F–k you” because he was so buttoned-up.

While the pair were in sync on a lot of things, there was an underlying “tinge of class rivalry to their gibes.”
Foer describes the pair as “the lunch-pail cornball’’ — Biden — “and the effete professor” — Obama — “culturally chafing each other.”

“Biden told a friend that Obama didn’t know how to say ‘f–k you’ properly, with the right elongation of vowels and the necessary hardness of his consonants,” Foer writes. “It was how they must curse in the ivory tower.”

Biden was concerned Obama, 62, wasn’t being tough enough, including in Afghanistan, listening too much to his advisers without pushing back.

Biden had advised Obama to rebuke his generals’ advice on surging troops in Afghanistan, whispering to his boss soon after the pair took over the White House, “These generals are trying to box in a new president’’ and “Don’t let them jam you,” according to the book.

Biden and Obama reportedly developed a “secret code” for when the vice president needed to play bad cop during high-level meetings, according to the book.

“When Obama tipped back in his chair at meetings, Biden took that as a cue to ask provocative questions that Obama wanted answered but didn’t want to raise himself for fear of shifting the tenor of a meeting,” Foer writes.

Once Biden took over the White House as president, he pushed the envelope hard to shake things up fast — with everything from the economy to the end of the war in Afghanistan.

“If Obama erred on the side of underreaction, Biden was going to err on the side of overreaction,” Foer says.

Biden’s hubris played a large role in his actions — and inactions — during the withdrawal and evacuation from Kabul in Afghanistan.

Biden overestimated his own competence in foreign affairs ahead of the fall of Afghanistan to the Taliban, making unhelpful and impractical suggestions while displaying a “swaggering faith in himself.”

It left his administration unprepared for the devastating chaos of the evacuation of Kabul.

“Biden had developed a theory of how he would succeed where Obama failed. He wasn’t going to let anyone jam him,” Foer says. “With his lifetime of experience, he had a plan to resist the pressures to stay.
“America didn’t know how to win the war in Afghanistan, but Biden knew how to win the bureaucratic argument.’’

Relying on his five decades in politics, Biden rejected advice and viewed experienced diplomats and pundits as “risk adverse, beholden to institutions [and] lazy in their thinking.’’

The State Department began operating under the Biden administration’s devastatingly false prediction that it would retain an embassy in Kabul after troops left.

That assumption would lead to grave and deadly problems in the withdrawal’s final months and aftermath.