Brooks has always been a Republican...but never a conservative. Before he wrote for the New York Times, he wrote for the Boston Globe.
He represents the "country club liberal Republican" set. Rush has spoken of him in this vein many times.
The apostates among the Beltway conservative punditry that he refers to are the Fred Barnes-types, not the David Brookses.
Over the past 15 years of listening, I've heard Rush refer to the Country Club Republicans many times, mostly with regard to Arlen Spector and other R politicians and sometimes those in the press. I have understood the notion of "Country Club Republicans" since the days of Reagan's first campaign for the presidency (Reagan was NOT a country club Republican) and understood why Reagan's presidential victory was atypical and an anomaly long before I began to listen to Rush. Rush's frequently mentioning them (the country clubbers) as a problem to conservatives, only increased my admiration for him, because it was a strong point of agreement. I never heard Rush mention Brooks in that regard, but I haven't listened daily for years (and have gone weeks not being able to listen) like I once did and wouldn't be surprised.
Thanks for the explanation.