You gotta remember that it's not about how many people you have locked up safely in your camp, it's how many more people you can draw in with them. If there were that many party-line Republicans out there then there wouldn't have been a Democractic supermajority as recently as a week ago. If all they're doing right now is drawing their more moderate supporters away from the edge, then that has the net effect of not increasing their support in any way while making it that much harder to talk to the swing voters without seeming batfuck crazy.
Having said all that, I don't think it's actually happening that way anyway. True, the Tea-Party Republicans are becoming louder with every passing day, but that's because they know they're becoming largely irrelevant even within their own party. Most of the non-incumbent Republicans that have been winning nominations lately have been right-leaning centrists at worst. Hell, McCain won his nomination almost purely on his reputation for not being too much of a conservative nutball, and it would have turned out great if his party hadn't somehow convinced him to shackle himself to The Ultimate Teabagger just to make up for it.
It's not that the GOP is eating itself so much as it already ate itself long ago, and now it's got indigestion.