President Trump wrote on social media on Sunday night that he no longer considered valid the pardons his predecessor granted to members of the bipartisan House committee that investigated the Jan. 6, 2021, attacks on the Capitol, and a range of other people whom Mr. Trump sees as his political enemies, because they were signed using an autopen device.

There is no power in the Constitution or case law to undo a pardon, and there is no exception to pardons signed by autopen. But Mr. Trump’s assertion, which embraced a baseless right-wing conspiracy theory about former President Joseph R. Biden Jr., was a new escalation of his antidemocratic rhetoric. Implicit in his post was Mr. Trump’s belief that the nation’s laws should be whatever he decrees them to be. And it was a jolting reminder that his appetite for revenge has not been sated.

“The ‘Pardons’ that Sleepy Joe Biden gave to the Unselect committee of Political Thugs, and many others, are hereby declared VOID, VACANT, AND OF NO FURTHER FORCE OR EFFECT, because of the fact that they were done by Autopen,” Mr. Trump wrote in a post on social media on Sunday night. “In other words, Joe Biden did not sign them but, more importantly, he did not know anything about them!”

The use of autopen is an ordinarily uncontroversial aspect of governance; it was first used to sign a bill into law at the direction of a president in 2011, when former President Barack Obama was traveling in Europe and wanted to sign a piece of legislation that Congress passed extending the Patriot Act another four years.

After Mr. Trump posted about the autopen and the pardons Sunday night, a reporter in the traveling press pool on Air Force One asked him to elaborate, and he seemed to briefly back away from the extraordinary idea he had just posted.

Would other things Mr. Biden signed as president using an autopen also be considered null and void, he was asked.

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