President Biden was peeved. What was Chuck Schumer thinking?

The Democrats had just temporarily averted a national default with Republican aid but still needed a broader deal to resolve a debt ceiling clash. Yet there was Mr. Schumer, the Senate Democratic leader, on the floor bashing Republicans for playing “a dangerous and risky partisan game.”

Mr. Biden called Mr. Schumer to chide him. That was not helpful, the president said, according to an official informed about the call, which came late in Mr. Biden’s first year in office. Senator Mitch McConnell, the Republican leader, had backed down to help avoid a fiscal crisis. They should not rub his nose in it. Mr. Schumer pushed back. “You don’t know how much he’s been beating up on me,” he told the president.

The Joe Biden who will defend his presidency at a nationally televised debate on Thursday night remains a practitioner of old-school politics in a new-school era. The hostility, the anger, the polarization, the “beating up” that define today’s national debate, yes, he knows all about that.

But after more than half a century in Washington, he still has the instincts of a backslapping cloakroom pol, eager to make deals and work across the aisle where possible at a time when that rarely seems rewarded anymore.

In some ways, it has been a formula for success that upended expectations, resulting in a raft of landmark liberal programs that will mark Mr. Biden in the history books as one of the most prolific legislative masters since Lyndon B. Johnson. And yet it has not been a formula for executing the most essential mission that he assigned himself when he took office: healing a broken country riven by profound economic, ideological, cultural, political and geographic divisions.

No president in American history took the oath with more experience in public office than Mr. Biden, 81, who was first elected to the Senate in 1972, when two-thirds of today’s Americans were not even born. But the politics of 2024 are a far cry from those of the 1970s, 1980s, 1990s, 2000s or even 2010s. While building a new bridge or lowering the cost of insulin still matter, they only penetrate so far with the electorate in an era of tribal animosity, populist unrest and social media disinformation.

“These days, people like to belittle experience, but his experience was just vital to getting as much done as we did,” Mr. Schumer said in an interview. Although he said he did not recall the 2021 phone call with Mr. Biden, he did not deny it either, and agreed that it sounded like the president’s approach.

“Biden was always of the view and understood instinctively that we had to do things in a bipartisan way,” Mr. Schumer said, adding that he concurred.

Yet that approach has not won over the public. Mr. Biden can travel the country, cutting ribbons from the most ambitious infrastructure package since the 1950s; he can trumpet the biggest investment in fighting climate change in history; and he can boast of job creation, unemployment and stock market figures that Ronald Reagan would have coveted in 1984’s morning in America. But polls show a majority of voters are not impressed or not paying attention.

Among many Americans, he is blamed for wars started by other countries that national security veterans nonetheless credit him with navigating maturely despite their own criticisms. He has not found a message on the economy that resonates more than the price of milk and eggs. Neither his persona nor his vision travel on today’s hyperactive, hypersonic, hyper-sensational social media the way that former President Donald J. Trump’s do. TikTok voters are not checking Mr. Biden’s legislative scorecard.

“It doesn’t mean a lot now because we’re in such a performative age,” said former Representative Eric Cantor of Virginia, who was a main Republican negotiator with Mr. Biden when he was vice president. “He’s just not equipped in this era we’re in of social media and constant scrutiny. The tools and attributes and talents that he may have had just don’t quite fit or lend themselves to the era we’re in.”

It was not supposed to be quite this hard. To be sure, Mr. Biden understood the enormous tests he faced coming into office — a global pandemic still killing thousands every week, an economy in collapse, schools and businesses shuttered, racial turmoil in the streets and troops deployed around Washington after an attack on the Capitol meant to overturn an election.

Somehow, though, Mr. Biden expected the fever to break as his twice-impeached and seemingly discredited predecessor faded away. The trick was that Mr. Trump refused to go away and has spent nearly four years stirring the pot, fueling resentment and trying to tear down the system that Mr. Biden represents.

And now, saddled with the lingering effects of inflation and images of migrants streaming over the border, Mr. Biden feels the weight of an election like no other. At stake in his mind are not just health care and tax policies or support for Ukraine and Israel. Grandiose as it sounds, he believes it is nothing short of democracy. It is America, as he sees it.

“It’s a huge burden,” said Jon Meacham, the historian and informal adviser to the president, “but Biden is a stand-in, the embodiment, pick your image, for the politics that we grew up with and that have shaped us since the New Deal.”

‘He Feels the Burden’

After 36 years in the Senate and eight as vice president, Mr. Biden thought he understood the presidency as well as anyone. But being a senator or even a vice president is nothing like being president, as Mr. Biden has come to learn, according to interviews with dozens of his friends, aides, cabinet officers and congressional allies.

“Even with all of that extraordinary experience, particularly being vice president for eight years,” Secretary of State Antony J. Blinken said in an interview, “there is something unique about being president. The way I’ve heard him express it to me on a couple of occasions in the Oval Office was standing behind the desk and sort of tapping the desk and saying, ‘The buck really does stop here.’”

When it came time to sit behind the Resolute Desk himself, Mr. Biden brought lessons from Barack Obama’s tenure. He would not lowball the size of his economic stimulus plan to jolt the country out of its Covid-induced recession, as he believed Mr. Obama had done 12 years earlier. Nor would he let the generals talk him out of withdrawing American forces from Afghanistan, as they did Mr. Obama.

Never mind the experts who warned against overstimulating the economy for fear of unleashing inflation or pulling out of Afghanistan too abruptly at the risk of abandoning allies to the Taliban. Mr. Biden’s mind was made up. And so there he was, sitting in the Situation Room, his head shaking no, his eyes closed in prayer, as Gen. Mark A. Milley kept updating reports of American troops killed by a suicide bomber at Abbey Gate at the Kabul airport.

Mr. Biden does not shrink from bad news, aides said. On his first full day in office, he pulled aside Jeffrey D. Zients, then his Covid response coordinator and now his chief of staff. “Inevitably there will be ups and downs,” Mr. Zients recalled him saying. “But tell me when there’s a problem. Put it on the table and together, we can solve it.”

And in the end, Mr. Biden showed up the doubters who told him not to bother trying to reach across the aisle. “People said this is so quaint, he actually believes we can legislate,” said Senator Chris Coons, Democrat of Delaware. “He got bill after bill that he signed into law that were bipartisan, that really pulled us together.”

But after a long career marked by insecurity, Mr. Biden still has a chip on his shoulder, insisting he knows the issues better than anyone around the table. “None of you have been elected,” he tells aides.

In trusting his instincts, Mr. Biden is not always receptive to contrary counsel on issues he feels strongly about. As one former aide put it, Mr. Biden wants an adviser to be a “cardboard cutout” who nods at whatever decision he makes. Another former official observed that “everyone is afraid” of Mr. Biden’s temper.

The decisions that critics often blame on his advisers, like slow-walking sophisticated weapons transfers to Ukraine to avoid escalation with President Vladimir V. Putin’s nuclear-armed Russia or continuing to support Israel despite concerns over civilian casualties in Gaza — those were all Mr. Biden. And he is willing to own them.

“He was extremely well prepared” for the job, said Ron Klain, his first chief of staff, “but he would be the first person to tell you there’s a difference between being the last person in the room giving advice and the person making the decisions. He feels the burden of that very heavily. They’re hard decisions. They’re close decisions. You definitely see it on his face and in his countenance. It definitely wears on him, but he’s borne up very well under it.”

‘Staying on Script’

To the extent that the presidency has changed Mr. Biden, it has made one of Washington’s most famously off-the-cuff politicians into a far more disciplined figure. Once loquacious to the point of being mind-numbing, today Mr. Biden eschews the long-winded stemwinders that made even his friends roll their eyes.

He holds fewer news conferences and grants fewer interviews than any president in decades, though lately he has picked up the pace, and often gives reporters one-word answers where he used to give a thousand. “The biggest change has been his caution and precision about staying on script and sticking to the points he wants to make,” observed Jay Carney, who was his communications director when he was vice president.

Still, in private, Mr. Biden prepares for public appearances in advance with long meetings, turning a one-hour session into two hours, sometimes punctuated by meandering stories about long-dead senators. He still has a fetish for ensuring speeches avoid jargon and relate to folks in Scranton, Pa., the onetime hometown he treats as his archetype for everyday Americans.

“He started nearly every meeting saying, ‘Would you please speak in straight English?’” recalled Cecilia Rouse, former chair of his Council of Economic Advisers. Or he picks up the phone to call Gina Raimondo, his commerce secretary. “Hey Gov,” he says, referring to her former post as governor of Rhode Island. “Help me put this in English.”

Those who have known him over the years said he remains at heart the same person. “What the American people see every day is who he is,” said Mr. Blinken, who has worked for Mr. Biden for more than two decades. “There’s no artifice. There’s no public versus private persona.”

But many, including some on his own team, assume the limits on his public interactions are meant to protect him from age-related mistakes. There has been simmering discontent within his administration among those who think the president’s inner circle goes too far in shielding him from public exposure.

“Everything looks and feels so choreographed, scripted and controlled that it doesn’t afford him the opportunities to show off his strengths — humor, empathy and compassion,” said Michael LaRosa, a former press secretary for Jill Biden. “Relatability is his superpower, but you can only relate to someone when their humanity is exposed. That means flaws, mistakes and everything else that comes with imperfection.”

Age is the radioactive nuclear rod of the Biden presidency that no one wants to touch despite the danger of leaving it unaddressed. That flows from the top. Defensive and testy, Mr. Biden deeply resents discussion of his age, and his closest aides have taken their cue from him. Rather than acknowledge the obvious issues while still highlighting the advantages of a seasoned president, some advisers argue that he has not slipped at all.

His shuffling walk, low voice and occasional confusion are hard to deny, even though Republicans exaggerate and distort them. In a recent Time magazine interview, Mr. Biden said “Europe” when he meant Russia, “Russia” when he meant Ukraine and “Putin” when he meant Xi Jinping. At times, he catches such mistakes right away and corrects himself, as many people of all ages do; at others, he does not seem to notice, much like Mr. Trump on occasion.

But looking past the verbal miscues and painful elocution, Mr. Biden does not wander into unreality the way that Mr. Trump, 78, often does and his substantive points are for the most part as conversant and informed as in the past. He exhibits a grasp of his policy choices and no one has cited a decision that he would have made differently if he were a decade younger.

“His age is what it is, right?” Ms. Raimondo said. “If you look at him, he is physically different than he was 10 years ago.” But “he has a wisdom and a judgment and a temperament and rock-solid core values that have made him one of the most effective presidents we’ve ever had.”

In meetings, aides said, he still gets to the heart of the matter. “When you sit down to talk with him, you really want to be prepared because he’s going to ask a lot of probing questions,” said Dr. Anthony S. Fauci, who served as his medical adviser. “You don’t wing it with him.”

Representative James E. Clyburn, Democrat of South Carolina, acknowledged that Mr. Biden does not seem like the animated lawmaker of old, but attributes that to a conscious decision to play it safe.

“He is a bit more subdued as president than he was as a senator,” Mr. Clyburn said. “People attribute that to age. Well, I’m older than Joe Biden by almost, I think, two years, but I don’t think that’s what it is. I think Joe Biden made some calculations some time back and changed his style a bit.”

‘Pain and Loss’

If the president seems more subdued, less the happy warrior than he once was, there are reasons apart from age and calculation. Mr. Biden, whose private family tragedy has long shaped his public career, has never been the same since his son Beau died of brain cancer in 2015.

Beau was the golden child, the military veteran and state attorney general envisioned by his father as the heir apparent of a new political dynasty. Mr. Biden had imagined this would be Beau’s presidency, not his. “You can’t overestimate the impact of Beau,” said Mr. Meacham. “He thought he passed the torch and then the torch was passed back.”

To this day, one ally said he practically never talks with Mr. Biden without Beau coming up. In part because of that, this has been something other than a joyful presidency. The last few years have been marked by trauma: the deadly pandemic and resulting economic calamity, the schisms in society, two wars, mass shootings. Each morning’s intelligence briefing documents a relentless cascade of crises. “What else do you got?” the president asks with dark humor.

And then of course there are the drug addiction and legal travails of his surviving son Hunter, who was just convicted of federal gun charges. As several advisers put it, there is “no lightness” in this White House. “It always goes back to pain and loss,” ruminated one aide. “Pain and loss.”

Perhaps no one in modern American life, however, has transformed pain into purpose more than Mr. Biden.

He has made Beau into his political touchstone, citing his son’s death to connect with voters who themselves endure suffering and tapping his own grief to drive him forward long after many would have retired.

He has few strong interests other than politics and family. While he exercises daily and goes to church weekly, Mr. Biden does not publicize his basketball picks or his latest Spotify music list, as Mr. Obama does. His staff could not name any movies or television shows he has watched lately other than assuming it would be something Jill Biden would have picked. He has been spotted carrying books on societal disruption by Fareed Zakaria, Heather Cox Richardson and William Ury, but rarely cites something he has read.

He is still rooted in Delaware. If he cannot return by train every night as he did in the Senate, he flies there many weekends on Air Force One. He prefers the house in Wilmington, while Jill Biden might rather go to the beach house in Rehoboth, but either offers comforts he does not find in the stately halls at 1600 Pennsylvania Avenue. He makes a point of calling each of his children and grandchildren nearly every day.

Sitting in beach chairs just 15 yards from the water’s edge last summer with his friend Mark Gitenstein, a longtime aide and now ambassador to the European Union, Mr. Biden kept jumping up when someone walking by called out, “Hey, Joe!” He often knew them and would start telling stories while Jill Biden and Mr. Gitenstein gave each other here-we-go looks. “He gets energy from people,” Mr. Gitenstein said.

Unlike the diffident Mr. Obama, Mr. Biden never met a rope line he did not want to work. Mr. Klain would make sure to leave time after a White House event so the president could linger and mingle. “That’s not a waste of time,” Mr. Klain said. “That’s what gives the president his energy and enthusiasm and you’ll find he gets back to the Oval Office and be three times more energetic.”

‘Hardened His Resolve’

Whether he has the energy for another four years to finish out a presidency at age 86 remains a central issue for the election. But while Mr. Biden talked during the 2020 election of being a transitional figure, he is not ready to transition yet.

Rather than be a caretaker, he has pursued a far more ambitious agenda, boosting the semiconductor industry, providing debt relief from student loans, expanding health benefits to veterans exposed to burn pits and codifying federal recognition of same-sex marriage. “It turned out the pragmatic thing to do was also to be bold and transformational,” said Pete Buttigieg, the transportation secretary shepherding his infrastructure law into reality.

But as proud as Mr. Biden is of those accomplishments, he knows his legacy will turn largely on whether he keeps Mr. Trump out of the Oval Office. If he went through any kind of organized process to consider the pros and cons of seeking re-election beyond talking with family, no one near him knows about it or is willing to say. Around the 2022 midterm elections, he told aides to assume he was running unless he told them otherwise. At some point, the conversation simply turned to the logistics of setting up a campaign.

Allies like Mr. Clyburn and Mr. Coons encouraged him to run again. While some senior administration officials privately expressed concern about the decision, none said in interviews that they had volunteered doubts to Mr. Biden directly. “You become president, why would you serve just one term?” asked Ted Kaufman, his longtime aide and friend. “You get the whole thing up and running, you get the experience, you do the hiring and you have this wonderful machine and you walk away?”

That may always have been fanciful. Mr. Trump made it unthinkable. To Mr. Biden, his predecessor represents a singular threat to American democracy. One aide said Mr. Trump’s decision to run again “hardened his resolve” to run too. “I fully take him at his word that in the absence of the threats to democratic norms, he would have let the torch pass,” Mr. Meacham said.

At a fund-raiser last year, Mr. Biden said Mr. Kaufman had warned him that another campaign could get “pretty ugly.”

“It wasn’t an automatic decision about running again,” Mr. Biden, then about halfway through his term, told donors. “Not because I didn’t think there was more to do, but because I thought to myself, you know, four more years — I mean, six more years — is a long time.”

Still, Mr. Kaufman said in an interview that even knowing the political costs, the president did not struggle with the decision. “No, no, no, no, no,” he said. “There was never ‘I’m thinking of not running for a second term.’”

Mr. Coons said two factors drove Mr. Biden’s decision — Mr. Trump’s apparent sympathy for Russia over Ukraine and his promise to consider pardoning everyone who stormed the Capitol. “All you need to know is the guy wants to abandon Ukraine and empower the Jan. 6 insurrectionists,” Mr. Coons said, “and you have your reason why Joe Biden decided to run again.”

All of which now rides on his shoulders. The consequences of losing haunt Mr. Biden. “I can say he certainly feels the weight of the world. He has said it to me,” said Ms. Raimondo. “He wears that burden every day.”

But he insists on doing it on his own terms. After five decades, he cannot suddenly become a practitioner of the kind of politics that Mr. Trump practices — the “beating up,” the vitriolic rhetoric, the toxic politics.

“I know there are people who think he needs to be more bombastic and he needs to be calling people names,” said Mr. Clyburn. “I don’t think so. I don’t think we ought to conduct ourselves running for office the way we don’t want our children to conduct themselves on the playground.”

That’s a debate Mr. Biden thinks is worth having.