Come on... Many Republican senators start out very conservative then become less so. Take a look at Senator McCain's ACU rating--- it's still pretty high because it used to be great. ACU ratings also don't always tell the whole story--- I remember Chuck Hagel getting a 100 score one year even though he'd spent it undermining the president from the left on conservative issues.
My only point is that folks just can't label a senator as a RINO, simply because they go off the reservation on one or even two issues. It's short-sighted.