That’s part of it, and in addition, McCain reminds them of themselves — old, frail, and liberal-leaning.
It must be the heat or something, but as you get older down here your brain cells start dying and you start pulling to the left. Remember some of the oddball liberal things Barry Goldwater got into in his final years.
There maybe a lot of independents and democrats registered as republicans butting into the Republican Primary, just like in 2008.