Republican candidate John McCain was true on his word the second time around he told late night comedian David Letterman he would appear on his show and even presented his host with apologies Thursday.

David Letterman however was unforgiving, demanding answers from his guest from the very beginning.

“Can you stay?” he asked McCain, who had barely seated himself comfortably, of course referring to the politician’s notorious cancellation in late September. MSNBC’s Keith Olbermann, who replaced McCain last time, could be seen waiting in the wings. Oh what subtle stabbing.

Then again, Letterman has thoroughly jabbed, poked and parodied the Republican presidential candidate over the past weeks, evidently frustrated over being stood-up for Katie Couric.

Expecting some tough questioning, McCain replied, “Depends how bad it gets,” and joked that he had asked his son in the Marine Corps to FedEx him a helmet and flack jacket for the occasion.

The host then asked his guest what had happened and a contrite looking McCain said, several times, “I screwed up,” making even Letterman laugh, and compared the questioning experience with his time as a prisoner of war during the Vietnam War – “I haven’t had so much fun since my last interrogation.”

“Look at all the conversation I gave you,” he teased Letterman.

It was not all fun and games from here on though, as Letterman asked him questions that other interviewers have perhaps avoided…such as grilling him on his choice for running mate, Alaska Governor Sarah Palin.

Letterman asked McCain if he felt it his stomach that Palin would be prepared in case of another attack similar to 9/11, to which the candidate responded, “Absolutely. She has inspired Americans.”

Letterman then asked the candidate if he backed his running mate’s campaign claim that Democratic candidate Barack Obama “palled around with terrorists” and McCain answered that the Illinois senator needed to explain his relationship with former Weather Underground activist William Ayers.

Relentless, Letterman then inquired whether McCain had not had a relationship with Gordon Liddy, which appeared to fluster the politician. A commercial break followed, after which McCain reiterated that he knew Liddy and that the Watergate burglar had paid his debt to society by serving time in prison.

The TV host went further, asking McCain whether he understood that his relationship with Liddy could come under the same scrutiny as Obama’s with Ayers, to which the GOP presidential candidate replied that he, unlike Obama, had been completely open about his relationship.

Letterman drily noted that Obama was 8 and Ayers 29 at the time of their alleged “palling around” and asked what they did. “Are they double-dating, are they going to dinner, what are they doing? Are they driving across country?”

To which McCain replied, “Maybe going to Denny’s.”

John McCain has been on David Letterman’s show a dozen times over the years. In order to honor his rescheduled visit to the studio, the candidate rented a helicopter to fly to New York after his campaign airplane could not leave Philadelphia due to weather conditions.

On Sept. 24, McCain notoriously cancelled his appearance on The Late Show arguing that he would temporarily suspend his campaign because of the economic crisis. Hours later, he was being interviewed by Katie Couric, to Letterman’s unending chagrin.