Donald Trump claims Joe Biden's January 6 committee pardons are "void" because the former president signed them using an "autopen".
Presidents — including Trump — have used autopens for decades to sign documents and correspondence.
And one expert says that by raising the issue, Trump is only opening himself up for potential problems down the track.
Here's what we know.
What is an autopen?
An autopen is a device that mechanically replicates a person's signature.
A pen or other writing implement is gripped by the arm of a machine, which reproduces a signature after a writing sample has been "fed" to it.
It basically means prominent people — such as celebrities or politicians — can sign hundreds of documents each day without having to be directly involved, or even be in the same room.
According to Harry Melkonian, a practising lawyer and honorary associate at the United States Studies Centre, it's impossible to tell if a document has been signed by an autopen.
"It’s a nice, neat, authentic signature," he said.
What has Trump been saying?
Trump raised concerns about autopens on Monday, US time, in relation to pardons issued by his predecessor to the congressional committee that investigated the January 6 Capitol riot.
Biden issued the "pre-emptive pardons" at the end of his last term to protect relevant lawmakers and staff from any possible retribution from Trump.
But in a post on his Truth Social website, Trump claimed, in all capitals, the pardons were "VOID, VACANT, AND OF NO FURTHER FORCE OR EFFECT", because the then-president signed them with an autopen instead of his own hand.
"In other words, Joe Biden did not sign them but, more importantly, he did not know anything about them!" Trump claimed.
What evidence does Trump have?
Trump hasn't offered any evidence to support his claims, and neither has the White House.
When asked whether Trump had legal authority to void pardons signed by autopen, press secretary Karoline Leavitt questioned Biden's awareness of the pardons in the first place.
"The president was raising the point that, did the president even know about these pardons?" she said.
"Was his legal signature used without his consent or knowledge? And that’s not just the president or me raising those questions."
She went on to cite recent reporting by the New York Post that quoted two unidentified Biden White House aides who speculated about alleged abuse of the autopen during his tenure.
Pressed for evidence, Leavitt told the press corps at the daily briefing: "You're a reporter. You should find out."
Dr Melkonian says there is no evidence to support Trump's claims, but if pardons were being signed without Biden's knowledge, it would be "a constitutional failure".
While the Constitution's 25th amendment allows cabinet to relieve a president of authority, Dr Melkonian says this has never been implemented.
"Trump is really exaggerating the Biden situation," he said.
"The mere fact you disagree with what Joe Biden did does not mean he was mentally incompetent. There’s a big difference."
The Associated Press (AP) said a representative for Biden had declined to comment.
How long have presidents been using autopens?
Presidents have been using autopens for decades, according to Dr Melkonian.
"Donald Trump has admitted that he's using it," he said.
Barack Obama became the first president to use one to sign a law in May 2011, when he signed an extension to the Patriot Act.
Obama was in France on official business and, with time running out before the law expired, he authorised the use of the autopen.
But recently, some conservative voices have been critiquing Joe Biden's use of the device.
The Oversight Project — part of US conservative think tank The Heritage Foundation — said it had analysed thousands of pages of documents bearing Joe Biden's signature, and found most were by autopen.
The project's executive director, Mike Howell, also claimed some of Biden's pardon papers specified they were signed in Washington on days when he was elsewhere.
What are the laws around autopens?
There aren't any laws governing a president's use of an autopen.
A 2005 memo from the Justice Department said the president doesn't have to physically sign a bill for it to become law.
The Constitution also doesn’t specify that pardons must be in writing, and autopen signatures have been used before for substantive actions by presidents.
On whether pardons must be in writing or by the president's own hand, the US Court of Appeals for the Fourth Circuit has said the "plain language of the Constitution imposes no such limitation".
Meanwhile, a solicitor general's guidance on pardons issued in 1929 said "neither the Constitution nor any statute prescribes the method by which executive clemency shall be exercised or evidenced".
But Donald Trump, who often makes a point of signing executive orders on camera using his trademark marker pen, said autopens should only be used for "very unimportant papers".
"We may use it, as an example, to send some young person a letter because it's nice," he told reporters aboard Air Force One.
"But to sign pardons and all of the things that he signed with an autopen is disgraceful."
Can Trump void Biden's pardons?
No he can't, according to Dr Melkonian.
"He doesn't have that authority," he said.
"It would have to be the Supreme Court ... and I honestly do not think the court would try to go near it."
In fact, Dr Melkonian said most presidents avoided critiquing past presidents' pardons altogether.
"Because [they're] going to be doing the same thing," he said.
"I don’t know why Trump is doing this. All he’s setting himself up for is being questioned."
ABC/AP
