Jill Biden has always been ready to do what she thinks is the loving thing for her husband. Before now, that has typically been to fight. Joe Biden has been counted out many times, only to prevail with his wife’s unwavering support.

Now the monumental question before Jill Biden is, What is the loving thing to do when the president’s decline is unfolding in front of America and  he’s up against the toughest opponent he has ever faced—time?

The 73-year-old college professor’s steely support has been on display throughout the couple’s nearly half-century relationship. She was by his side in 1987, when accusations of plagiarism derailed his first presidential bid. In 2008, when his campaign faltered and he dropped out after the Iowa caucuses. And in 2020, when he turned in dismal showings in early states, leaving Democrats flailing for alternative candidates.

After the president’s disastrous debate against Donald Trump last week, Jill Biden again put up her fists. She dismissed her 81-year-old husband’s poor showing as an off night, declaring, “We are not going to let 90 minutes define the four years that you’ve been president.”

“She’s a warrior for him,” said Anita McBride, who served as chief of staff to Laura Bush when she was first lady. “I think that is what is worrying Dr. Biden the most. That his legacy would be that he has to pull out after all these years for a bad debate.”

Locking arms against the world has long been the Biden way. But scrutiny of that approach is intensifying as more Democrats call on the president to withdraw from this year’s contest, citing concerns he will lose to the former president. As President Biden pushes to keep going, some Democrats think Jill Biden might be the only person who could persuade her husband to exit from the race.

Elizabeth Alexander, communications director to the first lady, said Jill Biden was a supportive spouse, but not a political adviser. “As much as any husband-and-wife team make decisions together that impact their lives, they absolutely do, but as she’s said more times than I can count, politics is his lane. She supports his career, and he supports hers.”

Alexander added: “There’s an inherent tension for all first ladies—one that might be familiar to many women in their lives—you are supportive, but can’t be so supportive that your motives are questioned.”

Not in question is the unfortunate timing of Jill Biden’s appearance this week on the cover of Vogue, showing her dressed glamorously in a white Ralph Lauren dress. The August cover, headlined “We Will Decide Our Future,” irked lawmakers and donors already worried that the Bidens’ insular decision-making risks putting the presidency back in Trump’s hands. A source close to the first lady noted the cover date was set months in advance and said the profile was an opportunity to reach women voters.

In Michigan on Wednesday, the first lady kept up the fight. “Because there’s a lot of talk out there, let me repeat what my husband has said plainly and clearly: Joe is the Democratic nominee and he is going to beat Donald Trump, just like he did in 2020,” she told a crowd of supporters gathered for the opening of a Traverse City campaign office. “Are you ready to help me?”

Outside the event were two men holding handwritten signs that said “Step aside, Joe.”

‘Joyful moments’

The love story of Joe and Jill Biden began in the ashes of despair, hers from a challenging divorce, his from the tragic death of his wife and young daughter in a car crash in 1972. Three years later, he called her out of the blue after he saw a photo of her—in an advertisement in the Wilmington, Del., airport—and asked his brother to help get him a phone number.

She was 23, and he was 32. She grew up in suburban Philadelphia, the oldest of five sisters. In her 2019 memoir “Where the Light Enters: Building a Family, Discovering Myself,” she describes hers as an idyllic childhood, filled with climbing trees and catching fireflies.

They both have recounted how she said no repeatedly to him—a widowed Delaware senator with two young sons—before agreeing to marry. Jill Biden has said she loved him but wanted to be sure because of the boys. They were wed in 1977.

Speaking at the Democratic convention in 2008, the late Beau Biden (Joe Biden’s eldest son) described the union as a collective family decision after losing his mother and sister. “Five years later, we—my brother, dad and I—married my mom, Jill,” he said, as she smiled at him from the audience. “And they, together, rebuilt our family.”

More challenges followed. Two aneurysms gave Joe Biden a brush with death in 1988. The family lost Beau Biden to brain cancer in 2015. And they watched their younger son, Hunter Biden , struggle with drug addiction that gave way to high-profile legal woes. She has written that the younger Biden son is the one whose personality is the most like hers.

While Joe Biden’s political career has been higher profile, Jill Biden’s has also been central to her identity. She says her husband has cheered her on in her professional ambitions, including her desire to keep teaching English as a community-college professor while serving as second lady, when Joe Biden served as vice president, and now as first lady. “Teaching isn’t just what I do. It’s who I am,” she said at a recent event. “Joe understands this.”

During Biden’s decades in the Senate and then eight years as vice president to President Barack Obama , new joys unfolded, too. They had a daughter together, Ashley, and they forged traditions, such as family Thanksgiving sojourns to Nantucket. And Jill Biden revealed her softer side, such as a fondness for practical jokes.

In 2021, the first lady pranked reporters on her plane, donning a wig to disguise herself as a flight attendant named “Jasmine” and handing out Dove ice cream bars.

“I’ve always believed you’ve got to steal the joyful moments when you can,” she wrote in her memoir.

‘Emotional stoicism’

Jill Biden likes to tell a story about once challenging a boy in her suburban Philadelphia neighborhood who threw worms at her sister. She knocked on his door and punched him in the face. She tells the anecdote to make a point about facing bullies, specifically Trump. It reveals something about her, too.

Quieter than her husband and more private, Jill Biden prides herself on her toughness, her work ethic and what she calls her “emotional stoicism.” She admits to holding grudges on her husband’s behalf. She made certain no one ever saw her cry during her husband’s health issues decades ago, or when Biden dropped out of the 2008 presidential race. At a campaign rally in 2020, she physically blocked her husband from protesters storming the stage.

The lengths Jill Biden will go to for her family were apparent last month, when she interrupted her participation in D-Day commemorations to fly from France and back within 48 hours to attend Hunter Biden’s trial in Delaware on gun charges. The trial exposed a dark period of betrayal and substance abuse in the Biden family following Beau Biden’s death.

Some questioned the cost of the flights to taxpayers. The Democratic National Committee said it will pay the government for the cost of a first-class ticket. A source familiar with the arrangements said this was always part of the planning.

After Hunter was convicted of lying about his drug use when he bought a handgun, Jill Biden walked out with him, her face impassive, holding his hand.

‘She’s ride or die’

Over the years, Jill Biden has shown mixed feelings about her husband’s presidential aspirations. She mourned when he exited the 1988 Democratic primary. She famously opposed a run in 2004 by walking past his advisers in a bikini with “No” written on her stomach in black marker.

But when it came to a fifth presidential race in 2024, Jill Biden was all in. With another term, she reasoned, he could accomplish even more.

For months, she has campaigned in battleground states, fundraising and meeting voters. She urges the party to be tough. This week in Allentown, Pa., she courted Latino voters at a panel on Hispanic-workers issues. She didn’t mention last week’s debate.

One person close to the White House said such relentless stumping is Jill Biden’s way of supporting her husband. “She is not a very outwardly affectionate person,” the person said. “This is how she shows it. She’s ride or die.”

Insiders say the first lady often joins political-strategy meetings and wants to be read in on plans. In 2022, she complained to staff after her husband held an expansive, two-hour press conference—which included questions about his mental acuity and Hunter Biden—that she felt went on too long.

‘There’s a true belief in him.’

Political spouses frequently get blamed when things go wrong.

Before “Lady MacBiden”—a now-circulating criticism that references the power-hungry Shakespeare character—there was “Queen Nancy” Reagan, and withering criticism of Hillary Clinton . “It comes with the territory,” said McBride. “You’re going to be the president’s closest adviser. You’re the one that can be totally honest.”

Some say it is unfair for Jill Biden to take the heat for the campaign and the president’s decisions. “She should be defending her husband. She should be the biggest supporter,” said Michael LaRosa, a former press secretary for the first lady. “She shouldn’t be the one to make this decision for the Democratic Party.”

A big part of the Bidens’ life is in Rehoboth Beach, Del., where they frequently spend weekends at their beach house. Locals spot the couple riding bikes, perusing the shelves at Browseabout Books and popping into the restaurant Egg, where Joe Biden gets pancakes and bacon and Jill Biden’s go-to is the crab-topped eggs Benedict.

Egg owner Missi Postles and her manager husband, Michael Postles, this week served up a big helping of support for Jill Biden’s push to keep her husband in the race. “I think she is making the right call,” Michael Postles said. “I think he’s the best man that’s running right now.”

Missi Postles, wrapping silverware in cloth napkins, concurred. “When she’s in here, you can see that there’s a true belief in him,” she said.

Write to Catherine Lucey at catherine.lucey@wsj.com , Valerie Bauerlein at Valerie.Bauerlein@wsj.com and Scott Calvert at scott.calvert@wsj.com