A day after US President Joe Biden struggled through a 90-minute debate that only served to fuel voter concerns about his age and fitness, Jill Biden stood before well-heeled donors at a New York fundraiser and tried to explain what they had all witnessed.

"'You know, Jill, I don’t know what happened. I didn’t feel that great,'" the president had confessed, she told them. "I said, ‘Look, Joe, we are not going to let 90 minutes define the four years that you’ve been president.’”

It offered an early glimpse into the president's mindset and how he rated his debate performance, which was widely panned as a major blow to his campaign.

As doubts about Mr Biden's candidacy began to circulate, his closest adviser was unequivocal about whether he would step out of the race. "When he gets knocked down, Joe gets back up, and that’s what we’re doing today," Mrs Biden said.

The first lady has stood beside her husband throughout his decades-long career, from his time as a Delaware senator to becoming commander-in-chief, often serving as the decisive voice behind many of Mr Biden's political choices.

While the president often turns to his tight-knit family on big decisions, Mrs Biden is among a handful of top advisers who wield the most influence over the president and could ultimately help him determine whether it is time to step out of the race.

“It’s fair to call her Biden’s closest adviser,” veteran Democratic political strategist Hank Sheinkop told the BBC. “Family matters to him significantly and that makes Jill Biden’s role even more important.”

The president’s younger sister, Valerie Biden Owens, who served as his campaign manager during his years in the Senate, as well as his son, Hunter Biden, are also among his most trusted confidantes.

After the fallout from the debate, Mr Biden huddled with his family for a long-planned trip to Camp David, the presidential retreat in Maryland, where they discussed the fate of his campaign and urged him to keep fighting, according to the BBC's US partner CBS. Hunter Biden was among the most vocal family members urging his father to remain in the race, CBS reported.

But as Democratic anxiety over the 81-year-old president's physical and mental stamina has spilled into public view in recent days, many inside the party have looked to the first lady for any hint of wavering over his candidacy.

Instead, she has continued to hit the campaign trail, travelling to the battleground states of Pennsylvania and Michigan this week for a string of political and official events.

"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," Mrs Biden told supporters at a campaign event in Traverse City, Michigan, on Wednesday.

Mrs Biden's influence in the West Wing, however, is not unusual.

Nancy Kegan Smith, president of the First Ladies Association for Research and Education, said there are historic parallels between Mrs Biden and former first ladies.

"Most presidents depend on the uncoloured advice of their wives because that’s the person who is normally closest to them,” she said.

She pointed to Lady Bird Johnson, the wife of former President Lyndon B Johnson, who advised her husband - ultimately convincing him with a moving letter - to run for the White House in August 1964 after he became president following the assassination of John F Kennedy.

Four years later in 1968, she changed her opinion, telling him to not run for re-election. He listened, Ms Kegan Smith said.

Many in the Democratic Party are waiting to see if a similar scenario may unfold in the next month, placing a greater spotlight on Mrs Biden.

The first lady keeps a busy schedule. She is the first in the East Wing to keep a day job teaching English at a northern Virginia community college. When she is not teaching, she is often on the road campaigning for her husband.

“Most modern first ladies have been in the political game for quite a while and have been political sounding boards to their husbands,” Katherine Jellison, an Ohio University professor who studies first ladies, told BBC News.

The president proposed five times before Mrs Biden said yes, and the couple married in 1977, five years after Mr Biden lost his first wife and daughter in a car crash that also injured his two sons.

When he decided not to run for president in 2016, he told 60 Minutes "it was the right decision for the family". He cited his reasoning was in part because of the loss of his son, Beau, who died from brain cancer in 2015.

Mrs Biden specifically played a role in her husband's decision not to run for president in 2003, Ms Kegan Smith said, pointing to a scene described in the first lady's 2019 memoir, Where the Light Enters. In the book, she recalled lounging by the pool as Democratic advisers inside encouraged her husband to launch a campaign. Wearing a bikini, she wrote "no" on her stomach in magic marker and walked through the meeting. Biden did not enter the race that year.

But the first lady has also come under pressure in recent days, facing criticism after the presidential debate for praising her husband after his poor showing on the debate stage.

“Joe, you did such a great job. You answered every question. You knew all the facts,” she told him on stage at a post-debate rally in Atlanta. A clip of the exchange was widely mocked on social media.

Some Republicans have also seized on Democratic worry, laying blame on the first lady for Mr Biden's debate performance. Representative Harriet Hageman, a Republican from Wyoming, even accused Mrs Biden of “elder abuse” in a post on X, for “rolling him out on stage to engage in a battle of wits while unarmed”.

The Drudge Report, a conservative website, ran a headline on its front page immediately after the debate that read: “Cruel Jill clings to power.”

“It’s really unfair to put the burden on her. She’s his spouse. She’s not a politician," Michael LaRosa, her former press secretary, told The Hill. “It’s not up to her to save the Democratic Party."

Mrs Biden, meanwhile, has stressed that the president's bid for re-election will continue as the stakes in November are high.

“Every campaign is important, and every campaign is hard,” the first lady told Vogue for their August cover story. “Each campaign is unique. But this one, the urgency is different. We know what’s at stake. Joe is asking the American people to come together to draw a line in the sand against all this vitriol.”

That urgency is something the campaign is hoping she'll be able to convey to voters. In a statement to the BBC, the Biden campaign called Mrs Biden an "effective messenger" on the campaign trail.

“As a teacher, mom, and grandmother, she’s uniquely positioned to connect with key constituencies across the country and speak to the president’s vision for America,” the statement said.

Still, her steadfast support combined with White House dismissals of media reports that the president is weighing his exit have yet to tamp down growing uncertainty about the Democratic ticket. The fallout has triggered a backlash of Democrats, donors and some lawmakers publicly calling for the president's withdrawal from the race.

"Joe has been knocked down and counted out his whole life... When he gets counted out, he works harder. And that's what he's doing, but he needs your help," she told Michigan supporters on Wednesday.

"We don't choose our chapter of history, but we can choose who leads us through it," she added.

For Mrs Biden, that choice remains her husband.