The first presidential debate is done and the aftermath has not been good for the incumbent, Joe Biden.

Mr Biden’s primary goal on Thursday was to ease concerns about his age. Instead, the debate deepened them.

Some Democrat politicians and operatives reportedly texted CNN commentators with hopes that Mr Biden, 81, would step aside.

Some floated the possibility of going to the White House and publicly stating concerns about him remaining as candidate.

Could Mr Biden back out now? What would happen? And who could replace him?

The Democratic Party’s nominee will officially be chosen in August at the Democratic National Convention (DNC).

There, a candidate must win support from the majority of “delegates” – party officials who formally choose the nominee. Delegates are assigned to candidates proportionally based on the results of each state’s primary election.

This year, Mr Biden won almost 99% of the nearly 4,000 delegates.

According to the DNC rules, those delegates are “pledged” to him, and are bound to support his nomination.

But if Mr Biden were to drop out, it would be a free-for-all. There is no official mechanism for him or anyone else in the party to choose his successor, meaning Democrats would be left with an open convention.

Presumably, Mr Biden would have some sway over his pledged delegates, but they would ultimately be free to do as they please.

That could lead to a frantic contest erupting among Democrats who want a shot at the nomination.

It is worth adding that Mr Biden has so far given no indication that he has considered stepping aside.

This is an even less plausible scenario.

In the modern political era, a major national party has never tried a hostile takeover of the nomination and there is no evidence of any serious plans to try now.

However, DNC regulations do have some small loopholes that could, in theory, make it possible to push Mr Biden out.

The rules allow delegates to “in all good conscience reflect the sentiments of those who elected them”, meaning they could look to someone else if Democratic voters across the country turned against Mr Biden in large numbers.

Vice-president Kamala Harris would automatically take Mr Biden’s place if he were to step down during his presidential term.

But the same rules do not apply if Mr Biden pulls out as a candidate for November’s election race, and there is no mechanism in place that would give the vice-president an upper hand at an open convention.

Instead, Ms Harris would have to win the majority of delegates, just like any other candidate.

As she is already on the Democratic ticket, Ms Harris could certainly be favoured. But her relatively low popularity among the American public may dull that advantage.

Her net disapproval is now lower than Mr Biden’s and Donald Trump’s, according to FiveThirtyEight.

Several Democrats did try to challenge Biden this election cycle, including Minnesota Representative Dean Phillips and writer Marianne Williamson.

But both were long-shot attempts and neither are likely to rise to the top of any shortlist.

There is some speculation that California Governor Gavin Newsom or Michigan Governor Gretchen Whitmer could be substitutes.

But neither candidate has shown they have any interest in replacing Mr Biden.

“I will never turn my back on President Biden,” Mr Newsom said on Thursday in the debate spin room in Atlanta.

“I’ve spent a lot of time with him and I know what he’s accomplished in the last three and a half years. I know what he’s capable of and I know his vision and I have no trepidation.”