If you watch McCain, he'll stand near the desk (and the camera) and wait until everybody else has voted and then he'll vote; that way he can see if his vote is "needed" or not; if it's not, he can then "vote his conscious," you know?
I have heard that even though he has often promised to vote for a bill for one of his colleagues, if his vote isn't needed, he'll vote however he "needs" to vote and then walk off the floor without explaining to his "fellow friend" why he's screwing them.
McC values his dem friends but has no use at all for senators from his own party.