Joe Biden and Donald Trump face one of the longest presidential campaigns in US history, raising questions about the endurance of the oldest ever pair of nominees -- and of weary voters.

Concerns about whether two men born in the 1940s are ready for a lengthy duel in 2024 were thrust back in the spotlight when a bombshell special counsel report called Biden an "elderly man with a poor memory

Biden, 81, tried to fight back during an extraordinary televised speech from the White House, but ended up causing more alarm, as he called the Egyptian president the leader of Mexico.

The question now is whether the debacle could be a turning point for the US election or whether voters will simply buckle down for nine months of more of the same.

Usually at this stage, at least one of the parties is embroiled in a heated primary contest. Instead, Trump, 77, is all but locked in for the Republican nomination, while Biden, running as an incumbent, faces next-to-no opposition.

That sets the scene for a potentially even more bruising replay of their already savage 2020 clash. Not only the candidates will be the same but their main themes: Biden on Trump as a threat to US democracy and Trump mocking Biden as decrepit.

In some ways the campaign is exceptional. Trump is seeking to be the first defeated former president to come back for a second term since Grover Cleveland in 1892.

But while the rest of the world is closely -- and nervously -- watching, US voters are increasingly turned off by the choice on offer.

"They've seen them each for four years," said William Galston of the Brookings Institution.

A University of Massachusetts poll on February 5 showed 53 percent of people preferred Trump not to stand, and 57 percent said the same for Biden.

Speculation -- and for some, fantasies -- abounds about one or both of the two men pulling out, whether as a result of illness or in Trump's case, multiple criminal cases. However they both insist they are irreplaceable.

Grueling campaign

Thursday's devastating special counsel report and Biden's angry retort -- particularly in denying the claim that he could not remember when his son Beau died -- have supercharged those concerns.

Polls show voters aren't as worried about Trump's age, despite the tycoon being nearly as old and himself recently mixing up the leaders of Hungary and Turkey among other gaffes.

"Age is primarily an issue for Biden," said Robert Rowland, professor of political communication at the University of Kansas.

Biden needed to get out in the public eye to explain his policies but his staff were protecting him "because they know of his tendency to go off message," he told AFP.

"If he's running for president, he's got to satisfy the people that he has the cognitive skills and the strength, and the only way he does that is by being in unscripted settings where they can see him think and answer in real time."

A grueling campaign schedule could, paradoxically, see Trump and Biden spend less time on traditional media.

Biden is set to skip a traditional Super Bowl interview before America's biggest sporting event, while TV debates between the two candidates are still up in the air. Trump has avoided any debates with his Republican rivals.

If voters are as indifferent as polls suggest, the 2024 battle is likely to depend more than ever on which candidate can best mobilize supporters.

Biden has recently turned his focus to turnout, stepping up appeals to Democrats to vote. For Trump, the battle will be to expand from his ultra-loyal hard-right base to the waverers in the middle who may not like either man.

© Agence France-Presse