ABC News' Jan Simmonds Reports: "I screwed up," Sen. John McCain told David Letterman this afternoon when discussing the subject of his last minute "Late Show" cancellation a few weeks ago.   

When McCain walked out to his seat on the show, Letterman jokingly asked, "Can you stay?" McCain jokingly replied that he had asked his son, who is serving overseas, if he could send over his helmet and flap jacket, but it "didn't arrive on time.

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

Letterman, not satisfied, asked McCain about why he didn't go right back to Washington like he said. Again McCain implored, "I screwed up. It's only Dave Letterman, there is only a few million people who will be watching."

When Letterman asked if Palin was ready to become president if events forced that to occur, McCain emphatically answered, "yes."