McCain has health and age working against him.
McCain's health seems to be fine, and he's younger than Reagan when he ran for his second term.
I don't think either one of those will be a factor. I also think a not insignificant number of Democrats would vote for McCain.
He'd get almost all the military votes in a match up with Hillary.
That's right he had nose cancer!
That fits.
McCain is known for cutting off his nose to spite his face.