If McCain's approval is at 40% he's in the danger zone. This is not a good year to be an incumbent. That mostly goes for Democrats, but Republicans aren't much more popular. If McCain had kept on good terms with conservatives and the folks who would become Tea Partiers he wouldn't be in trouble, but the latter part of his career has been spent sticking his finger in their eye.
He might win, but I wouldn't put my own money on it right now.