I think it's a bit different if he'd just gotten elected based on running on your platform - then he's obviously of lesser character.
Of course, conservative Southern Democrats walk, talk, and act like Republicans and the switcheroos in Georgia may have just been biding their time before they switched to the party that truly represented their beliefs. And, no doubt, they probably campaigned on a conservative platform that did not resemble the national Democratic party's platform.
The worst you can call them, therefore, is opportunistic.