Oh boy. Looks like the long-friendly relationship between Sen. John McCain and David Letterman may be on the skids.

The late-night comedian was none too pleased that McCain decided to bail on an appearance on the “Late Show” tonight so he could rush back to Washington to deal with the economic crisis –- particularly since he stopped and gave an interview to CBS News anchor Katie Couric along the way.

The Republican presidential nominee was supposed to make his 13th appearance tonight on Letterman’s show, the venue where he originally announced his White House bid in 2007. But he backed out late Wednesday afternoon, saying he was suspending all campaign activities to focus on getting a financial bailout plan through Congress.

During the taping of tonight's show, Letterman told the audience that McCain had called him personally to apologize for standing him up and said he was rushing to the airport to get back to Washington. The late-night host called McCain “an honest-to-God hero,” and questioned why he needed to halt his campaign. Couldn’t his running mate, Sarah Palin, take over while he was in Washington, Letterman asked.

“I’m more than a little disappointed by this behavior,” he said. “We’re suspending the campaign. Suspending it because there’s an economic crisis, or because the poll numbers are sliding?”

The audience whooped and applauded.

“You don’t suspend your campaign,” Letterman added. “Do you suspend your campaign? No, because that makes me think, well, you know, maybe there will be other things down the road –- if he’s in the White House, he might just suspend being president. I mean, we've got a guy like that now!”

Things got worse for McCain when Letterman, in the midst of interviewing substitute guest Keith Olbermann, learned that the GOP candidate was actually still in New York. Not only that, but McCain was about five blocks away at the CBS News headquarters, sitting down for a last-minute exclusive interview with Katie Couric.

Incredulous, Letterman interrupted his chat with Olbermann to show the audience a live shot on the internal CBS news feed of McCain getting touched up by a makeup artist as he waited to talk to Couric.

“He doesn’t seem to be racing to the airport, does he?” Letterman said, shouting at the television monitor: “Hey John, I got a question! You need a ride to the airport?”

More highlights, including Letterman's Top Ten Questions People are Asking The John McCain Campaign after the jump...

-- Matea Gold

Photo: An earlier appearance by John McCain on "The Late Show with David Letterman." Courtesy CBS.