Joe Biden: Could the Democrats replace him as US election ...

Joe Biden's stumbling performance in the US presidential debate has sparked alarm among Democrats - but could he be replaced against his will and how might that work?

A national US party has never tried to force an election candidate to step down in the modern era and their rules make it almost impossible to do so.

The issue came before both parties in 2016, but neither took action and there's currently no known effort to force Mr Biden to give way.

It would mean Democrat officials overturning the results of the state primaries - the votes that took place earlier this year that confirmed Mr Biden as the overwhelming pick for November's election.

The 81-year-old won virtually all the delegates - and they are now set to go through the customary process of rubberstamping his nomination at the Democratic National Convention on 19 August.

There are ways to replace a nominee if they die, resign or are incapacitated, but forcing Mr Biden out would mean delegates choosing another candidate.

It appears highly unlikely, as they were chosen in the primaries because of their loyalty to the president and with the trust that they would vote for him at the convention.

Read more:'Unmitigated disaster' for Biden in TV debate with TrumpKey moments from Trump v Biden

However, there is a potential "loophole" in that Democrat rules allowing them to "in all good conscience reflect the sentiments of those who elected them".

So if concerns about his performance and mental acuity reach critical level this could be a way forward.

Watch a special programme with reaction to the US presidential debate on The World with Yalda Hakim on Sky News from 6pm

What if Biden withdraws himself?

This is the only plausible scenario for the Democrats to choose a new candidate to take on Donald Trump.

Mr Biden has repeatedly dismissed this option during other turbulent times, but if he changed his mind he could simply serve out the remaining months of his presidency.

Such a U-turn would create a frenzy among Democrats as there's no protocol in place for him or the party to choose a new candidate before August's convention.

Read more:Excruciating Biden performance was among worst everThe presidential debate as it happened

It takes a majority of the party's 4,000 or so delegates to win the presidential nomination - and Mr Biden secured about 3,900 of them in the primaries.

The president would have some influence over his pledged delegates, but ultimately they can vote as they please - so it could become a 'free for all' with candidates campaigning aggressively to win them over.

If the president decides to call it quits after the convention takes place, the chair of the party can call a special meeting of around 500 members.

They can - in theory - choose a new nominee via a simple majority vote.

However, such a process would likely be far more turbulent with behind-the-scenes jockeying and public campaigning.

If Mr Biden pulled out even closer to the 5 November election, it could raise constitutional, legal and practical concerns.

For example, voting papers have to be printed well in advance and it might not be possible to change them in time.

Would vice president Kamala Harris replace Biden?

If Mr Biden were to resign immediately as president, Kamala Harris would automatically succeed him - but would not also become Democratic nominee.

She might be politically favoured if he ditched his re-election bid, but Mr Biden's delegates would not automatically transfer to her and it would be a level playing field with other hopefuls.

Who could be favourite if Biden steps back?

Two frontunners - if they chose to put themselves forward - could be California governor Gavin Newsom, 56, or Michigan governor Gretchen Whitmer.

They didn't enter this year's primaries to compete against Mr Biden but have both been talked about as potential Democrat nominees in future.

For now at least, Mr Newsom appears to be sticking firmly behind the president.

He told reporters after Thursday's debate his party "could not be more wholly unified behind Biden" and he shouldn't step aside.

Mr Newsom has been California governor since 2019 and was also San Francisco's mayor for seven years. Before that, he founded a winery in the state's upmarket Napa Valley.

Ms Whitmer, a lawyer and former prosecutor, has previously said she has no interest in replacing Mr Biden in this year's election - but could she now have a rethink?

The 52-year-old was the Senate's first female Democratic leader and became governor in 2018.

She also served as co-chair of Joe Biden's 2020 election campaign.

Democrats including Minnesota's Dean Phillips and Jason Palmer, who actually beat Mr Biden in the American Samoa, could also try their hand if Mr Biden pulls out.

They were his main rivals in the primaries but only got a couple of delegates each, so would likely struggle to pull in enough support.