(AP Photo)

I caught Sen. John McCain tonight on the David Letterman Show on CBS, taking his medicine for cancelling a similar appearance two weeks ago as part of McCain’s short-lived “suspension” of his presidential campaign to deal with the national financial meltdown. Letterman was upset by the last-minute disappearance. But he got downright angry when he learned during the taping of that show that McCain hadn’t actually left New York for Washington, but instead went a couple of blocks over for an interview with CBS news anchor Katie Couric.

Letterman got a lot of comedic mileage out of the faux pas with a relentless pounding of McCain for several nights. But I wondered if Letterman would back off with McCain rescheduling and actually showing up this time. Nope! Most of Letterman’s jokes in the monolouge were fired right at the Arizona senator. The funniest was a camera shot into the wings where MSNBC Countdown anchor (and huge McCain critic) Keith Olbermann was waiting to jump in if McCain didn’t show again (Olbermann filled in for McCain two weeks ago).

Then, Letterman gentled harangued the senator once he got on stage. At least McCain didn’t dodge his responsibility. When Letterman asked for McCain’s explanation, he said simply, “I screwed up.”

McCain likely expected a quick joke or two at his expense and then by-gones would be by-gones. Instead, he was in for a long night. Once Letterman finally let the canceled appearance drop, he grilled McCain over and over about selecting Alaska Gov. Sarah Palin as the Republican vice presidential candidate. Letterman’s questions honed in on Palin’s experience, or lack thereof, and Letterman’s conviction that she’s not prepared to lead the country through another terrorist attack like 9/11.

McCain did take the opportunity to give the longest and most detailed justification I’ve heard from him about Palin. McCain’s list of Palin’s strengths include her reputation as a reformer, her growth from a PTA mom to mayor and then governor, her willingness to challenge (and defeat) an incumbent from her own party when she saw her state going in the wrong direction, and her huge popularity in Alaska today.

In all, it seemed like rather painful experience for McCain. But he took his lumps and he probably won’t have Letterman hounding him for the next 18 days of the campaign.