On Saturday afternoon, US Vice-President Kamala Harris sat on stage at a black cultural festival in New Orleans, talking about her life story and what she felt she had achieved in the White House.

It was the kind of event that the first female, black and South Asian American vice-president has regularly attended throughout her three-and-a-half years as Joe Biden’s deputy, usually trailed by a small press pack dwarfed by that which follows the president himself.

But as panicked Democrats a thousand miles away in Washington weighed replacing 81-year-old Joe Biden as the party’s candidate for November’s election following his woeful and sometimes incomprehensible debate performance against Donald Trump, the number of reporters trailing Ms Harris had swelled to dozens.

On stage and through her travels this weekend, the vice-president did not address swirling questions about Mr Biden’s fitness for office and whether he should withdraw and hand the baton to her.

But in discussing ambition and how to forge your own path with her audience in New Orleans, she encouraged the crowd not to listen to naysayers.

“People in your life will tell you, though, it’s not your time. It’s not your turn. Nobody like you has done it before," she said. "Don’t you ever listen to that."

Since the disastrous CNN debate on 27 June, she has repeatedly defended her boss, arguing that his record as president shouldn’t be outweighed by 90 minutes on a debate stage. Mr Biden himself has struck a defiant tone and fiercely insisted that he will remain the nominee.

Yet as calls grow louder for the president to step aside, some high-profile Democrats are unifying behind 59-year-old Ms Harris as the natural candidate to replace him.

On Sunday, congressman Adam Schiff of California told NBC's Meet The Press that either Mr Biden had to be able to “win overwhelmingly or he has to pass the torch to someone who can”. Kamala Harris, he added, could “very well win overwhelmingly” against Trump.

That’s a proposition that has raised eyebrows among some Democrats, including Biden allies, who see in Ms Harris a vice-president who failed in her bid for the 2020 Democratic nomination before the first ballot was even cast and who has struggled with an uneven record and low approval ratings throughout her time in the White House.

Against that, senior Democratic lawmakers like Mr Schiff and South Carolina congressman Jim Clyburn have been floating Ms Harris as the obvious successor should Mr Biden ultimately bow to party pressure.

Supporters point to a handful of polls that suggest she would perform better than the president in a hypothetical match-up against Donald Trump, and they argue she has the national profile, campaign infrastructure and appeal to younger voters that could make the transition seamless four months before election day.

An elevation to the top of the ticket would be a remarkable turnaround for a woman not long ago seen as a political weakness by senior figures in the Biden White House. Even Mr Biden himself reportedly described her as a “work in progress" during their first months in office.

But Jamal Simmons, a longtime Democratic strategist and Harris’s former communications director, said she had long been underestimated.

“Whether she’s a partner to the president or she has to lead the ticket, she is somebody who Republicans and the Trump campaign need to take seriously,” Mr Simmons told the BBC.

Since the debate and its fall-out, Ms Harris has altered her schedule to stick close to the president. She appeared at a heavily-scrutinised meeting last Wednesday where Mr Biden sought to reassure powerful Democratic governors about his fitness for office.

And a day later, on the Fourth of July - America’s Independence Day - she abandoned her usual tradition of grilling hotdogs for firefighters and Secret Service agents at her Los Angeles home to be by Mr Biden’s side at the White House celebrations.

The former top prosecutor has focused on criticising Trump in public appearances since the debate, pressing the case as to why voters should believe he is a threat to democracy and women’s rights. At the same time, she has offered nothing but steadfast support for Mr Biden.

Vice-presidents always need to strike a delicate balancing act between ambition and loyalty, but Ms Harris knows that this is not a moment where she can show any daylight between her and the president.

Kamala Harris is, however, far from the only alternative to Mr Biden being discussed. The list of potential Biden replacements ranges from a cadre of popular governors - Gretchen Whitmer of Michigan, Gavin Newsom of California, Pennsylvania’s Josh Shapiro and Illinois’ JB Pritzker - to Transportation Secretary Pete Buttigieg and California congressman Ro Khanna.

Ms Harris and her staff have refused to engage in public speculation. But her team is keenly aware of the behind-the-scenes conversations taking place as some party members coalesce behind her.

A memo circulated online, purportedly written by Democratic operatives, laid out a detailed argument to promote Ms Harris despite her “real political weaknesses”.

Trying to choose anyone other than her would throw the campaign into disarray and keep "Democratic bickering" in the media spotlight for months, it argues.

If Mr Biden were to give up the nomination, the idea of the Democrats passing over Ms Harris in favour of another candidate appalls many on the left of the party and in its powerful black caucus.

In that situation, "this party should not in any way do anything to work around Ms Harris", Mr Clyburn, one of the most prominent black lawmakers in Congress, told MSNBC last week.

Republicans, too, have acknowledged Ms Harris would be the frontrunner to replace Mr Biden.

Senator Lindsey Graham of South Carolina warned on Sunday that Republicans must be ready for a “dramatically different race” should Ms Harris - whom he described as a “vigorous” candidate - become the nominee.

Mr Graham emphasised her progressive California brand, suggesting she was closer in policy terms to left-wing firebrand Bernie Sanders than Joe Biden, in what appeared to be a glimpse of a Republican attack line should she become the candidate.

For his part, Donald Trump has called her “pathetic” in the days since the debate.

But ultimately the only question that matters for many Democrats - including deep-pocketed donors - is if she has a better chance of beating Trump than Joe Biden does. And that is deeply uncertain.

Harris backers point to a recent CNN poll suggesting she would fare better than the president against Trump in November. In a head-to-head contest, Ms Harris trailed the Republican by only two points, while Mr Biden lagged six points behind him. The poll also suggested Ms Harris performed better than Mr Biden with independent voters and women.

But many polling experts dismiss such hypothetical surveys, noting voter sentiment would change if Mr Biden actually decided to step aside and the Democrats entertained other potential candidates.

One Democratic pollster close to the Biden campaign acknowledged that Ms Harris may have more potential to expand the party’s voter base than the president, but was sceptical about how much of a difference she would make. Surveys pitting her against Trump at this stage “don’t mean anything”, said the person, who requested anonymity because they were not authorised to speak to the media.

Ms Harris, the child of an Indian mother and Jamaican father, performs better in surveys than Mr Biden with black, Latino and young voters - critical constituencies that allies say she could energise as the nominee.

But whether she would actually boost turnout among younger voters of colour is another uncertain question. “This is just a wait and see moment,” the pollster said.

Some in the party are also asking whether Ms Harris’ progressive reputation risks losing the union and blue-collar voters in the battleground states of Pennsylvania, Michigan and Wisconsin that Mr Biden narrowly won in 2020 and which both parties need to secure a win in November.

Should she take over the ticket, some Democrats have suggested that Governor Josh Shapiro of Pennsylvania or Governor Roy Cooper of North Carolina could be picked as running mate to capture centrist voters in Midwestern states.

Given the ages of Joe Biden and Donald Trump, voters are paying far more attention to the VP candidate of both parties in this election cycle, said Celinda Lake, a veteran Democratic pollster who worked for the 2020 Biden campaign.

On the Republican side, Trump has yet to announce his running mate, although many speculate he’ll pick North Dakota Governor Doug Burgum or Ohio Senator JD Vance.

Deep concerns among some Democrats about Harris's strength as a presidential candidate date back to her unsuccesful 2020 bid for the party's nomination, in which she landed blows on Mr Biden in an early debate but then crashed out before the first caucuses in Iowa.

Critics said she struggled to define herself as a candidate, a sentiment that has lingered throughout her tenure as vice-president. She had a shaky start in the White House, marked by high-profile interview slip-ups, low approval ratings and staff turnover.

She was also tasked with overseeing the administration’s strategy to reduce migration over the US southern border, which increased to record levels over the last three years and remains a major vulnerability for the campaign.

Those early stumbles led Ms Harris to be more cautious about her public appearances but many voters perceive her as ineffective and absent. “People need to know more about her, what economic issues she is strong on and they need to be reminded of the role she’s played,” Ms Lake said.

Over the last year, Ms Harris has found stable footing as the administration’s leading voice on abortion rights, an issue that proved successful for Democrats during the 2022 midterm elections and one the party hopes will win back more voters in November.

As a former prosecutor who handled sexual violence cases, she has invoked personal stories of working with women who miscarried in the bathroom or were turned away at hospitals as she’s tried to mobilise voters around the issue.

On the campaign trail, she has also sought to capitalise on other issues that resonate with young voters, including student debt forgiveness, climate change and gun violence. The White House, too, has made a concerted effort to promote her more forcefully.

Still, she faces an uphill battle to change longstanding voter scepticism - her approval ratings hover around 37% in polling averages compiled by FiveThirtyEight - a level similar to both Mr Biden and Trump.

And unless Mr Biden himself caves to the mounting party pressure to step down, grassroots Democratic supporters themselves seem resigned to supporting the current ticket.

At the Essence festival in New Orleans, Iam Christian Tucker, a 41-year-old small business owner from New Orleans, said she didn’t care, ultimately, who the nominee was.

She said she liked Kamala Harris, but she wasn’t sure if a black female president could win election.

“I’m voting against Donald Trump more than anything,” she told the BBC.

Greg Hovel, 67, who attended a rally for President Biden in Madison, Wisconsin, last week, said he supported Ms Harris in the 2020 primary and “has always been a fan,” though he cautioned there is “a lot of anti-woman sentiment in this country.”

“I think she would make an excellent president,” Mr Hovel said. “But I still think Biden can win.”

Mike Wendling contributed to this report from Madison, Wisconsin