It's his pro-life stance causing him the problem - the internals of the Rasmussen poll showed that Santorum's standing about Republicans increased on a second ask after being informed that Casey was also pro-life.
If Chaffee loses to a more conservative Republican in the primary then that nominee will deserve maximum support. If Chaffee does not lose that primary, then the Republicans of that state preferred him and he will be rightward of the alternative in the general election for One Critical Reason.
That being that he casts the most important vote of any session correctly -- the vote for majority leader. It is the majority leader that defines the agenda and what bills reach the floor.
The Kean situation is disappointing. He had a lead recently.