If President Joe Biden decides to drop out of the Democratic Party race, what happens next?

Joe Biden, America's 46th president, is the presumptive nominee of the Democratic Party to run for a second four-year term in the election to be held on November 5 this year. 

His nomination is due to be formalised with a roll call vote at the Democratic National Conference in Chicago in mid-August.

After announcing last April he would seek a second term, Biden had no major challenger for renomination during this year's Democratic Party primary process, where he won every state and territory bar one — American Samoa (where Biden lost by 11 votes out of just 91 cast).

Needing 1,968 delegates from those state and territory primary and caucus elections to seal the Democratic nomination, Biden won 3,896.

After almost half a century in Washington politics; 36 years as US senator, eight years as vice president and four as president, Biden should be in an impregnable political position at least within his own party.

He is not.

A terrible debate performance two weeks ago unleashed a wave of panic among supporters that at 81 Biden is simply too old to beat Trump, much less serve as president until January 2029.

A growing number of Democratic members of congress, including the first senator, Peter Welsh of Vermont, have publicly called for Biden to step aside. Prominent supporters including actor George Clooney have added their voices to the growing but still muted chorus.

Others such as former speaker Nancy Pelosi have recently been giving Biden less-than-full support; "It's up to the president to decide if he is going to run," Pelosi told NBC.

"We're all encouraging him to make that decision, because time is running short."

Pelosi's comments were intriguing, because as far as we know Biden has decided to stay in the race, he's said so many times in the past two weeks. 

The former speaker and longtime Biden ally seems to think that's not the end of the matter.

So, if Biden were to decide to drop out, what would happen next?

There is no simple answer to that question, which in part is why there hasn't been greater pressure on Biden to leave the race.

It could get very messy.

Unlike when a president is incapacitated or dies, and the vice president assumes the presidency, the constitution says nothing about replacing a presidential candidate, other than that to be elected they must be at least 35 years of age, a "natural born citizen" and "fourteen years a resident" of the United States.

So, what happens next if Biden quits will come down to the rules of the Democratic Party, which state:

"All delegates to the National Convention pledged to a presidential candidate shall in all good conscience reflect the sentiments of those who elected them."

The assumption has been that all 3,896 delegates Biden won in the primaries would vote for him, but if Biden has withdrawn, what do they do?

The tidiest solution would be for Biden to "release" his delegates on the understanding that they would support his Vice President Kamala Harris.

But the party rules require delegates to examine their conscience based on the "sentiments" of 16 million Democrats who voted in the primaries, mostly for Biden, but not directly for Harris, who was not on the ballot.

Delegates and party leaders could instead decide to open up the nominating contest, as they commonly did before the 1970s and conduct ballots to see which candidate can secure the 1,968 votes to become the nominee.

Then we could see any number of governors, senators, even actors like George Clooney try and put themselves forward for nomination.

Clooney 2024?

It's not impossible.

The last time something akin to that happened, coincidentally in Chicago in the year 1968, following Lyndon Johnson's decision not to run, and leading contender Robert Kennedy's assassination, there were fist fights on the convention floor and a police riot against anti-Vietnam war protesters on the streets. 

In the end, vice president, Hubert Humphrey, who didn't stand in a single primary, won the nomination due the the backing of party leaders, but lost the following election.

If Biden stays in the 2024 presidential contest, most experts agree it would be almost impossible to take the nomination away from him at the convention. 

Rules can be changed, but not easily.

However, in theory, Democratic party rules make it much easier to remove the nominee after the convention, but before the presidential election in November.

"Filling a Vacancy on the National Ticket: In the event of death, resignation or disability of a nominee of the party for president or vice president after the adjournment of the National Convention, the National Chairperson of the Democratic National Committee shall confer with the Democratic leadership of the United States Congress and the Democratic Governors Association and shall report to the Democratic National Committee, which is authorised to fill the vacancy or vacancies."

So the 200-member Democratic national committee – made up of governors, senators, former presidents and other party notables, headed by chair Jamie Harrison – can simply replace Biden due to death, resignation or disability. Would they declare Biden "disabled" due to age or signs of failing cognition as opposed to suffering a major health episode? Would Biden resign himself if he has another bad interview, press conference or flubs the next debate in September?

Unlikely, but again, not impossible.

The last time anything like that happened was in 1972 when vice presidential nominee Tom Eagleton resigned as senator George McGovern's running mate after revelations about his electro-shock treatments for depression created an outcry about his fitness for office.

In all conceivable scenarios, including the DNC removing Biden, Vice President Harris is the most likely replacement. It is hard to see a political party which depends on female and non-white voters to win, overlooking the first woman or person of colour to serve as US vice president.

But these are unpredictable times, and what happens next is anyone's guess — maybe even President George Clooney.

John Barron is co-host of ABCTV's Planet America and Fireside Chat programs. He is also the author of a modern history of American presidential elections "Vote For Me!". Planet America returns July 17.