US President Donald Trump has ordered an investigation into what Republicans claim was a "conspiracy" to cover up Joe Biden's cognitive decline during his time in the White House.

Wednesday's move, which was slammed by Biden, is the latest in a long-running campaign by Trump – with the backing of Republican Party politicians and their cheerleaders in the conservative media – to discredit his predecessor.

But it also comes as a growing chorus of Democrats begins to acknowledge the former president appeared to have been slipping in recent years.

Those concerns were thrown into stark relief by a disastrous debate performance against Trump during last year's presidential campaign, in which the then-81-year-old stumbled over words and repeatedly lost his train of thought.

"In recent months, it has become increasingly apparent that former president Biden's aides abused the power of presidential signatures through the use of an autopen to conceal Biden's cognitive decline," a presidential memorandum reads.

"This conspiracy marks one of the most dangerous and concerning scandals in American history.

"The American public was purposefully shielded from discovering who wielded the executive power, all while Biden's signature was deployed across thousands of documents to effect radical policy shifts."

Biden vehemently denied the allegations.

"Let me be clear: I made the decisions during my presidency," he said in a statement.

"I made the decisions about the pardons, executive orders, legislation, and proclamations.

"Any suggestion that I didn't is ridiculous and false."

Biden slammed the probe as "nothing more than a distraction by Donald Trump and Congressional Republicans who are working to push disastrous legislation".

The Democratic Party is increasingly riven by squabbles about whether Biden could have been forced to step down earlier to give the party chance to find a more popular presidential candidate.

Biden's former White House press secretary Karine Jean-Pierre on Wednesday criticized the in-fighting by the Democrats, calling it a "betrayal" of Biden and announcing her departure from the party as a result.

The fight has been given oxygen with the publication of a book by journalists Jake Tapper and Alex Thompson that claims the former president's inner circle connived to keep him from public view because of his decline, which included forgetting familiar faces like Hollywood star and party stalwart George Clooney.

Trump's claims of a cover-up were also boosted by news that Biden is suffering from an "aggressive" prostate cancer, with some voices on the right insisting – without evidence – the diagnosis must have been known some time ago to those close to the former president. (AFP)