Even after Mr. Letterman brought out Keith Olbermann, the MSNBC host and vituperative Republican critic as the substitute guest for Mr. McCain, he continued to assail Mr. McCain for the decision to cancel the appearance. His critique reached a high point when he learned that at the very moment Mr. McCain was supposed to be on the couch next to him being interviewed, the senator was at the CBS News center three blocks away in Manhattan, getting ready to be interviewed by the CBS News anchor, Katie Couric.

Mr. Letterman ordered his director to put on a live feed from that location, which showed Mr. McCain getting made up to go on with Ms. Couric. “He doesn’t seem to be racing to the airport,” Mr. Letterman observed.

After listening to some questions from Ms. Couric, Mr. Letterman said, “Hey, John, I’ve got a question: You need a lift to the airport?”