President-elect Donald Trump and President Joe Biden are seen ahead of inaugural ceremonies on Jan. 20. (Tom Brenner/For The Washington Post) By Perry Stein and Matt Viser

President Donald Trump on Wednesday ordered a broad investigation into whether Joe Biden covered up a cognitive decline while in the White House and was incapable of executing presidential decisions — an unprecedented request that in theory could undermine thousands of Biden’s executive actions and pardons.

The order focused in part on whether Biden’s use of the autopen — a machine that uses real ink to duplicate a human signature — on executive actions was legitimate, or whether his aides used it to hide a president who was not cognitively capable of signing them himself.

Presidents have long used autopens to si

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