McPartyHopper tried twice to become a Dumpocrat.
The 1st time, according to those who've spoken, he met with Ol' Dashie (I think) to see if he could keep his comittee leadership position. It might have worked, but Jumpin'Jim beat him to the deed and the Doobie-crats didn't need Ol' Oatmeal Sides to flip the power structure of either of the houses. If you think back, you'll remember the JumpinJim incident had an air of hurried desporation to it. Almost as if someone didn't want to have to garuntee McSillyputty anything...
The 2nd incident was more obvious: Ol' Oatmeal Sides (gee, I like that one!) started whining around about being available for Kerry's VP pick, talking-up the historosity of both parties "'burying their differences to be on the same ticket". He even made a couple of public statements on how absoluely neat it would be for a Dumbocrat & Regurgiton to be on the same ticket -- only a true Pair O'Mavericks would do it! There was a well-publicized meeting between McSillyputty & Le French Hound Dog... (maybe 2...).
Kerry eventually turned him down. No one has leaked why.
Maybe you need to get things in perspective. You are talking about fairly recent events. McCain ran for Congress as a Republican in 1982, so that's when, at the ;latest, he made his party choice.
I still say he's a natural Dem, but the anti-military sentiment of the Dems from the 1960s forward would have been difficult for McCain to stomach with his background.
You are talking about events of the 21st century, after he'd been in Congress about 20 years.