`` I screwed up , '' John McCain tells David Letterman for canceling scheduled visit . Letterman used McCain 's no-show as subject of ongoing joke . Comment:
`` I screwed up , '' John McCain tells David Letterman for canceling scheduled visit . Letterman used McCain 's no-show as subject of ongoing joke . McCain also discussed Sarah Palin , Obama 's ties to 1960s ' radical William Ayers .