That's when it occurred to the former "Late Show" host that he's missing out on a lot of comedic material. So he briefly stepped back into the spotlight Friday, joining funnymen Steve Martin and Martin Short onstage during their show in San Antonio, Texas, to give a Top 10 list called "Interesting Facts About Donald Trump."
Letterman took aim at The Donald's hair, his anti-immigrant comments and his general knuckleheadedness: "Thanks to Donald Trump, the Republican mascot is also an ass."
In the video above, the Top 10 begins at about the 4:30 mark, but Letterman's introductory banter with Martin and Short is worth watching, too.
Welcome back, Dave, if ever so briefly.