Actually, I give Chaffee more credit than that. He is from a total left-wing state and thus he has good reason to be moderate, although I would agree that he goes too far.
Hagel, McCain, Warner, and Graham have no reason at all to be playing footsie with the Dems, and I'd love to see them get tossed out in their next Primary (and they very well might if we lose this nuke vote because of them).
When a Senator chooses to identify with the Republican party, than during the election year announces he will not vote for their presidential candidate (when he could just keep his mouth shut if he prefers not to give endorsement) I give him no credit at all. It is one thing to be a Liberal Republican. It is another to use the Party to be elected, than disregard the Party to be for all intent a Democrat.
At that point the cord need to be cut.