A new poll released on Sunday found that 86 per cent of American voters believe that 81-year-old Joe Biden is too old to serve another term.

In a report released on Thursday, special counsel Robert Hur described the US president as a “well-meaning, elderly man with a poor memory”, and thus ruled he should not face further scrutiny over allegations he mishandled confidential documents.

Despite growing evidence suggesting Mr Biden is an electoral liability, experts warn that ditching him as the Democrat nominee for the November presidential election is fraught with difficulty.

Mr Biden’s supporters have an iron grip on the Democratic National Committee, which has bent over backwards to support him.

It altered the primary calendar, decreeing that New Hampshire - where Mr Biden fared badly in 2020 - should no longer be the first in the nation, instead making it so that South Carolina - which brought his campaign back from the dead four years ago - was first on the calendar.

If the Democrats wish to now introduce a new candidate to run against Mr Biden for the nomination they would be severely inhibited as 80 per cent of the upcoming primary contests have closed applications for new entries.

Any rule changes to allow for a new candidate on the ballot in those states would have to be passed by the party’s Biden-backing Rules and Bylaws Committee.

In theory, delegates who have already pledged their support for Mr Biden but are now concerned by his mental state could defect when it came to a floor vote. But that is considered highly unlikely.

“There is no mechanism that could force him out. That could only happen voluntarily,” explained Christopher Galdieri, professor of politics at Saint Anselm College in New Hampshire.

“If he were to stand aside a lot would depend on timing. If it took place after the primaries,  when all the delegates had been selected they would make the decision at the convention.

“You would have a throwback to the 19th century where the nomination was fought out on the floor, rather than rubber-stamping the primary and caucus results.”

Professor Galdieri said if Mr Biden dropped out while the primaries were in full flow “There would be absolute chaos. Biden’s name would be on most of the ballots.

“In other states, there would be a scramble,” he added.

“Either way it would be the worst of both worlds, There would be very short notice.

“Candidates who have not been through the wringer would turn up at the convention and tear each other apart.”

Mr Biden remains publicly determined to contest the presidential election. If he opted to drop out, he would follow in similar footsteps to Lyndon Johnson who, in failing health and unnerved by Bobby Kennedy’s entry into the presidential race as well as the close contest posed by Eugene McCarthy, announced he would not seek re-election.

“I shall not seek and I will not accept the nomination of my party for another term as your president,” he said in an address to the nation in 1968.

Some of Mr Biden’s allies have suggested he should follow the example of Ronald Reagan’s riposte in a presidential debate when critics suggested that at 73, he was too old to seek a second term.

“I want you to know that also I will not make age an issue of this campaign. I am not going to exploit, for political purposes, my opponent’s youth and inexperience.”

Even his opponent, Walter Mondale, laughed.