The US president has claimed that aides took advantage of his "cognitively impaired" predecessor

US President Donald Trump has accused senior aides to his predecessor of committing "treason at the highest level" by allegedly exploiting Joe Biden's cognitive decline to implement unauthorized policies that, according to Trump, the former president would never have supported himself.

In a Truth Social post on Tuesday, Trump claimed that Biden never advocated for open borders and that the surge in illegal immigration was orchestrated by staffers who effectively "stole" the presidency by taking control of the autopen - a device used to replicate a person's signature.

"The Joe Biden that everybody knew would never allow drug dealers, gang members, and the mentally insane to come into our Country totally unchecked and unvetted. All anyone has to do is look up his record," Trump argued, adding that the resulting influx has cost the US "hundreds of billions of dollars."

"It wasn't his idea to open the border... It was the people that knew he was cognitively impaired, and that took over the autopen," he added. "This is TREASON at the highest level!... Something very severe should happen to these treasonous thugs that wanted to destroy our Country, but couldn't, because I came along."

Trump's allegations come amid renewed concerns about Biden's health, following the recent announcement that he has been diagnosed with an aggressive form of prostate cancer, characterized by a Gleason score of 9 and metastasis to the bone. While Trump publicly extended well wishes to Biden, he also questioned the timing and transparency of the disclosure, suggesting that the public was not informed soon enough.

"I'm surprised that... the public wasn't notified a long time ago, because to get to stage nine, that's a long time," Trump told reporters on Monday, questioning how doctors at the White House and Walter Reed hospital could have missed a condition that takes years to develop.

"And the autopen is becoming a very big deal. You know, the autopen is becoming a big deal because it seems that maybe it was the president - whoever operated the autopen," Trump added, speculating whether the same doctors had overlooked signs of cognitive decline. "In other words, there are things going on that the public wasn't informed of. And I think somebody's gonna have to speak to his doctor."

The autopen has become a focal point in Trump's broader criticisms of Biden's presidency. Earlier this year, Trump declared that several last-minute pardons issued by Biden were "void" because they were allegedly signed without his knowledge.

Legal experts note that the use of the autopen by US presidents has historical precedent and is legally valid when authorized. However, Trump maintains that in Biden's case, the device may have been misused by aides acting independently. House Oversight Committee Chair James Comer has since launched an investigation into whether senior Biden aides orchestrated a cover-up of the president's alleged cognitive decline.