A number of Democratic lawmakers are calling on President Joe Biden to end his re-election campaign after his botched debate against Republican challenger Donald Trump.

Two weeks later, more than a dozen House representatives, one senator and leading donors including actor George Clooney have joined the growing chorus. 

A series of opinion polls since the debate suggest Trump may be edging ahead in key states.

President Biden has continued to insist he's staying in the race, using an hour-long news conference to try to make his case. 

As the pressure builds, attention has turned to who could replace him if he changes his mind about running.

How could Biden be replaced?

Various scenarios could play out over the coming days and weeks, but there's two main ways Mr Biden's campaign could end: he could pull out willingly, or be forced to step aside by his own party.

The first option is more straightforward. 

Mr Biden has won the support nearly all of the Democratic delegates who will vote at the party's convention in Chicago in August. If he steps down, they would be released to vote for another candidate. Whoever is able to win a majority of delegates at the convention would be the new nominee.

The second option would be much messier. 

Democrats could change the rules and turn the convention into a free-for-all against the president's wishes. 

If President Biden did voluntarily suspend his campaign, he might at the same time throw his support behind a possible successor. 

Who would be the leading contenders?

Vice-President Kamala Harris

Vice-President Kamala Harris, who is already on the ticket, is an obvious and increasingly popular choice within the party to replace Mr Biden. 

As his deputy, she has become the face of the administration's campaign to protect reproductive rights after the Supreme Court overturned Roe v Wade. 

Ms Harris has proved to be a loyal ally to the president and fiercely defended his debate performance. After the event, she admitted the president had a "slow start" but argued he went on to provide more substantive answers than Trump. 

Days after the debate, as concern grew about the president's ability to stay atop the ticket, Ms Harris reiterated her support for Mr Biden. 

“Look, Joe Biden is our nominee. We beat Trump once and we’re going to beat him again, period,” she said Tuesday.

“I am proud to be Joe Biden’s running mate."

Ms Harris has the strong name recognition that comes from the job of vice- president, but has struggled with low approval ratings throughout her tenure. 

Fifty-one percent of Americans disapprove of Ms Harris, while 37% approve, according to polling averages tracked by FiveThirtyEight. 

Michigan Governor Gretchen Whitmer

Gretchen Whitmer, the two-term governor of Michigan, is an increasingly popular Midwest Democrat who many pundits speculate will run for president in 2028. 

She has campaigned for Mr Biden in the past and has not been shy about her political aspirations. 

She told the New York Times she wants to see a Generation X president in 2028, but stopped short of suggesting that she might fill that role. 

In 2022, she led a campaign that left Michigan Democrats in control of the state's legislature and the governor's mansion. 

That political control allowed her to enact a number of progressive policies including protecting Michigan abortion access and the passage of gun safety measures. 

California Governor Gavin Newsom

California Governor Gavin Newsom is one of the Biden administration's fiercest surrogates. He frequently appears on cable news networks praising Mr Biden. 

But Mr Newsom has political ambitions of his own. 

He is often listed as a possible 2028 candidate, but many Democratic pundits now say he could be a stand-in for Mr Biden. 

Mr Newsom raised his national profile in recent years by being a key party messenger on conservative media, and via a debate against Florida Governor Ron DeSantis last year. 

He was a top surrogate at Mr Biden's disastrous debate in Atlanta in June, and dodged several questions in the spin room about whether he would replace Mr Biden.

For now, he is publicly standing by the president. He travelled to Washington to attend a Wednesday meeting with Mr Biden and other top Democratic governors, and headlined a Biden campaign event in Michigan on the 4th of July holiday.

Transportation Secretary Pete Buttigieg

It is no secret that Transportation Secretary Pete Buttigieg has presidential aspirations. 

He ran for president in 2020 and is often touted as one of the Biden administration's best communicators. 

Mr Buttigieg has managed a number of public crises during his time as transportation secretary. 

He helped to oversee the government response to the East Palestine train derailment in Ohio, the Baltimore Bridge collapse and Southwest Airlines' scheduling crisis in 2022. 

Pennsylvania Governor Josh Shapiro

Pennsylvania Governor Josh Shapiro has seen high approval ratings since he was elected in 2022 in a swing state Mr Trump narrowly carried in 2016. 

The governor, who previously served as the state's attorney general, has worked across party lines during his tenure. 

He made national headlines last year after quickly rebuilding a collapsed bridge on a crucial Philadelphia highway - a major political victory for a first-term governor. 

The speedy repair was hailed by many as the perfect infrastructure talking point for a potential 2028 presidential candidate. 

Illinois Governor JB Pritzker

JB Pritzker, the governor of Illinois, has raised his profile in recent years by going after Trump and defending Mr Biden. 

The billionaire businessman - heir to the Hyatt hotel chain - is quick to post criticism of Trump on social media. 

After the debate he called Trump a "liar" and said he is a "34-count convicted felon who cares only about himself". 

Like Ms Whitmer, Mr Pritzker has a track record of completing agenda items on progressive Democrats' to-do lists on issues like abortion rights and gun control.

Other possible candidates? 

The list of potential nominees stretches beyond these Democrats, as the party has developed a deep bench of possible future presidential candidates. 

Kentucky Governor Andy Beshear, a two-term Democratic governor in a very conservative state, has earned growing national attention since his re-election last year. 

Maryland Governor Wes Moore found himself in the spotlight in recent months following the collapse of the Francis Scott Key Bridge in Baltimore. 

Senators Amy Klobuchar and Cory Booker have run for president in the past and have some name recognition among Democrats. 

Georgia Senator Raphael Warnock, who won a closely contested Senate race in a swing state, also has been mentioned as a potential replacement for Mr Biden. 

A Reuters IPSOS poll released Tuesday found the only person who could beat Trump in November was Michelle Obama. But the former first lady has repeatedly said she does not have presidential aspirations.