My initial reaction on hearing the news was that after generating a bunch of Democratic House seats, the Club for Growth has now produced its first Democratic senator. I assume that Specter’s votes will now move leftward.
There’s probably an Exit, Voice and Loyalty sort of point to make here. People in any group setting, whether a political party or a school, will often have two basic strategies at their disposal: Try to exert internal pressure to pull the group in a new direction, or defect and find (or form) a new group. These strategies are mutually reinforcing, since ability to exert influence within the group is enhanced by the threat of defection, at least on the assumption that (other things equal) people benefit to some extent from preserving the existing coaltions. As in so many other strategic contexts, the most effective defection threat is the one you can credibly commit to even when carrying out the threat is worse for you. So, in other words, Specter says: You’d rather have me than a Democrat, so toe the line. (A third party? Go ahead… throooow your vote away! Mwahaha!) The Club replies: Nope, we’ll vote for a conservative even if it sinks us in the general. They don’t actually want that result, naturally, but the point of a strong credible threat is that you don’t ever have to carry it out. The only problem is that a sufficiently strong candidate ends up with the opportunity to defect too. That leaves them in the worst possible outcome: Even if they’d carried out the threat, they could at least hope that with no incumbent in the race, their man would have a fighting chance. Against a popular incumbent, not so much—so the same candidate is returned to the Senate, but pushed to the left.
This is something of a general hurdle for a group that wants to pursue the Club’s strategy. Your ability to threaten to shift support to a primary challenger is a function of the candidate’s defiance of the base—he needs to have deviated enough that you can get people worked out about casting a ballot for someone else in protest. But an incumbent in a strong enough position to be exhibiting that level of defiance is probably in a strong enough position to be contemplating defection himself. Offhand, I’d guess the Club strategy would be more effective focusing on more vulnerable pols in their first few terms as a means of enforcing conformity on one or two super-salient issues. Of course, given that a fracas like this may just serve to delegitimize the Club in the eyes of conservatives, the point may well be moot for the foreseeable future.