I don't think it would have been that big of a deal if he would have gone back to DC to get in the mix but the hype that came after by the Fox was what made it a joke.
They blew it up..."OMG! OMG! McCain is suspending his campign...(giggle, giggle)." That was enough to get the joke rolling. Guess who came running into the studio breathless with a Cheshire cat grin on his face to announce it?...You guessed it, Cameron.
Killed momentum (By playing into the hands of the media, all too eager to ridicule the “knight in shining armor” routine) and the result of the futile action/vote killed his Presidency.