Rand Paul has taken a shot at Chris Christie, blasting him for not being conservative enough. This is a line of argument I've seen a lot of in recent years, especially during last year's Republican Primary. One candidate will slam another candidate for not being conservative enough. It's like being moderate and not driven by rigid ideology is a bad thing.
I've seen the Democrats slam each other for supporting things that Republicans have supported, like the War in Iraq. But I've never seen them accusing each other of not being liberal enough.