Rush says he's a "realist," who lives in "Literalville." IMHO, his reality has always had a very "Establishment" flavor. He is going with their conventional wisdom that Mitt Romney is the most "electable," whatever his ideological flaws... and again, he's going with the big money.
In "Literalville" Romney's ability to raise the cash really counts. In this scenario, Newt is getting in the way.
A primary election is the only election in which Rush, and the rest of the breed might have any real influence and IMHO, they are going with Romney.