Rush is a political commentator, but he is also running a business. He is smart to stay out of primary fights in the Republican party, and always has EXCEPT when the primary has someone who in no way can be conservative. Then, he doesn't hurt himself to criticize a Republican, because his audience is conservatives.
That is what he is doing, he is pointing out negatives on Huckster and McCain while remaining silent on everyone else. He has gotten frustrated enough lately that he has started to drop not so subtle hints for Thompson, and he is also saying nice things about Romney. I think he's concerned tht Thompson may not make it, and he is willing to reluctantly accept Romney (which is the situation with Ann Coulter, by the way).
In any event, Rush's refusal to get involved in a primary campaign is completely understandable. He wants to discuss the issues and frame the debate, which in turn will help candidates who agree with him, while not overtly supporting a particular candidate and alienating his audience. I can't argue with his decision. See what direction Hewitt's audience numbers are going, and let me know what you find out.
If a prominent conservative radio personality chooses not to use his influence to promote his ideology in time he will become irrelevant. We tune in to be inspired not just entertained.