I love a good celebrity feud! First John McCain ticked off Paris Hilton, and now he’s landed on David Letterman‘s bad side after cancelling his appearance on The Late Show last night.

Here’s the kicker: McCain showed up for a CBS Evening News interview with Katie Couric that same day. Ooh, diss! McCain claimed he was halting all campaign activity Wednesday in order to turn his attention to the nation’s financial crisis, and a McCain spokeswoman said the campaign felt “this wasn’t a night for comedy.”

Well, I think we can all agree that Katie Couric has completely lost her sense of humor since leaving Today on NBC, so I guess I can kind of see her point.

Not so Keith Olbermann, who subbed for McCain as a guest on Letterman last night and egged him on like a school chum as Letterman ripped into McCain. “This doesn’t smell right,” Letterman said. “This is not the way a tested hero behaves. Somebody’s putting something in his Metamucil.”

Letterman then showed video of McCain getting made-up for his appearance on the CBS Evening News: “Doesn’t seem to be racing to the airport, does he? This just gets uglier and uglier.”

Olbermann, to his credit, offered an explanation for the apparent diss in favor of Couric: “The other possiblity is that she has all the money that’s required to fix the economy.”