WASHINGTON — Mr Joe Biden's age has long threatened to be his political Achilles' heel — but a bombshell Special Counsel investigation suggesting the US president couldn't even remember when his son died has weaponised the issue in an election year.

Questions about the 81-year-old's mental acuity were already in the headlines after America's oldest ever president twice confused a world leader with a dead predecessor in the past week. 

Now Republicans have pounced on an issue that handily distracts from the fact that 77-year-old Donald Trump — the former president who's likely to face Mr Biden in a rematch in November — has made a few slips of his own.

A string of polls have showed that Mr Biden's age is one of the biggest concerns for US voters. He would be 82 at the start of a second term, and 86 at the end.

With a long campaign ahead, the issue will only get hotter, said Mr Robert Rowland, professor of political communication at the University of Kansas.

"He's got to satisfy the people that he has the cognitive skills and the strength," Mr Rowland told AFP.

The special counsel report is gold dust for Mr Biden's Republican foes, who swiftly used it to paper over their own chaotic political infighting.

Special counsel Robert Hur's report said Mr Biden would not face criminal charges over keeping classified documents — but his comments about Mr Biden's memory loss could be a major blow to his reelection hopes.

Trump-allied Republican House Speaker Mike Johnson blasted: "A man too incapable of being held accountable for mishandling classified information is certainly unfit for the Oval Office."

Mr Hur made repeated references to Mr Biden's "diminished faculties”, said he could not remember the dates of his vice-presidency under Mr Barack Obama, and could not remember "even within several years" when his son Beau died.

Beau Biden died of brain cancer in 2015.

"A 'well-meaning, elderly man with a poor memory' who has access to the nuclear codes..." Republican Congressman Kevin Hern said on social media, quoting from the report.

'GRATUITOUS'

A political veteran who began his career as a senator in 1972 and endured the tragic death of his wife and baby daughter in a car crash, Mr Biden has also long had a reputation for gaffes and word salads.

Those have multiplied in recent years, and together with a number of stumbles and falls that have gone viral on social media, questions have swirled about his readiness for a second term.

The White House pushed back hard against what it called "inaccurate, gratuitous, and wrong" criticisms of Mr Biden's memory.

"The report uses highly prejudicial language to describe a commonplace occurrence among witnesses: a lack of recall of years-old events," his lawyers wrote.

Even before the special counsel report the White House was already on the back foot, facing repeated questions at a briefing on Thursday over him confusing various European leaders.

On Sunday Mr Biden told a fundraiser that he had spoken to long-dead French president Francois Mitterrand, instead of current leader Emmanuel Macron, at a G7 summit in Britain in June 2021.

Mr Biden told a similar story about the same G7 meeting on Wednesday — this time saying he had met German Chancellor Helmut Kohl, who died in 2017, instead of Angela Merkel.

Mr Biden, who suffers from a lifelong stammer, also appeared tired while answering questions after a speech on the Mexican border crisis at the White House on Tuesday.

"This happens. It happens to all of us," White House Press Secretary Karine Jean-Pierre said.

Mr Biden didn't talk about the memory loss comments, but said the "matter is now closed" and pointed out that he had even agreed to an interview with prosecutors the day after the Oct 7 Hamas attacks on Israel.

He also said the report had contrasted the "stark differences" between his case and that of Mr Trump, who faces criminal charges over classified documents at his Mar-A-Lago resort in Florida.

Voters don't seem as concerned by Mr Trump's age but he has also had a few recent glitches, such as mixing up his rival for the Republican nomination, Ms Nikki Haley, with former US House speaker Nancy Pelosi. AFP