U.S. Sen. John McCain appeared on "The Late Show with David Letterman" Thursday night and admitted his failure to show up when he was previously booked was a mistake.

"I screwed up," McCain said. "But look at all the conversation I gave you."

After some ribbing, Letterman said he was willing to let bygones be bygones.

Small wonder: Letterman probably had an inkling he would attract a big audience -- his biggest, in fact, in nearly three years.

Letterman has hammered McCain since the Republican presidential candidate missed an appearance three weeks ago and the host questioned McCain sharply Thursday on running mate Sarah Palin and campaign tactics.

Nielsen Media Research says 6.5 million viewers watched, nearly 3 million more than the show typically gets. It was Letterman's biggest night since a December 2005 appearance by someone else he's had tense times with -- Oprah Winfrey.

McCain also apologized to "Joe the plumber" for all the attention he brought him, saying, "Joe, if you're watching, I'm sorry. But from what I've read, and I have not talked to him ... he's taken it pretty well."

Letterman asked if Palin objects when her supporters shout "terrorist" or "kill him" about Obama.

"She does. She doesn't countenance that kind of thing," McCain said. "I mean, nobody does. Sen. Obama doesn't. Joe Biden doesn't."

Letterman asked how Palin was picked as his the candidate's running mate.

"Well, we have this dart board ..." McCain joked.

Letterman continued to grill McCain on the Palin pick, saying, "If I were to run upstairs, wake you up in the middle of the night, and say, 'John, is Sarah Palin really the woman to lead us through the next four, eight years? Through the next 9/11 attack?' "

"Absolutely," McCain replied. "She has inspired Americans. That's the thing we need. We need inspiration now. We need courage. We need to know that we're the greatest nation in the world. And we can come through this."

Then Letterman brought up William Ayers and McCain's and Palin's attempts to link him to Obama, who was 8 when Ayers was a member of the radical Weathermen. McCain addressed this as he has in the past, then tried to swith the topic to the ecomony.

The unrelenting host asked McCain about a fundraiser he had attended at the home of Gordon Liddy, saying "that the same case could be made of your relationship with him as is being made with William Ayers."

"I know Gordon Liddy," McCain countered. "He went to prison, he paid his debt, as people do. I'm not in any way embarrassed to know Gordon Liddy. And his son, who is also a good friend and supporter of mine."

Letterman pressed on about the charge that Obama "pals around" with Ayers. "Are they double dating? Are they going to dinner? What are they doing? Are they driving cross country?" he asked.

"Maybe going to Denny's," McCain said. "Who knows?"

Read more in TV editor Rob Owen's Tuned In Journal.