My gut feeling is that Rush has a lot more "inside" information than he lets on. My guess is that is "neutral" comments about Specter was to convey some sort of message to the Specter people. Rush talked a lot about Bush's mandate, and that we are the majority and because of that we can tell Specter what to do. Rush still remembers the fiasco in 1995 when Newt Gingrich thought he had overwhelming public support, only to see everything blow-up in his face and ensure a second Klintoon term.
http://www.rushlimbaugh.com/home/daily/site_110904/content/across_the_fruited_plain.member.html
Thanks.
The problem was with Newt. Gingrich managed to alienate a lot of people on the right, a number being among those who worked with him. It doesn't pay to doublecross your friends and allies. Rush either doesn't know the story, or more likely doesn't choose to relate it.