There is only one time when I truly believed Rush was doing the Republican Party's bidding. When Specter (who Rush does not like and generally does not support) was running for relection to the Senate (the last time) Rush had Specter on his program just prior to the primary and not Specter's conservative challenger, TOOMEY, despite many requests from listeners to give Toomey equal time.
At the time, Rove and the President thought a Specter victory would help the President carry PA (and it didn't).
Rush had Specter on his program because Rush had said some things about Specter and Rush allowed Specter to come on and defend himself. Rush didn't endorse either candidate. Rush did not call Specter either, Specter called Rush's program and got bumped to the front when it was alerted to Rush that Specter was on the line to defend himself. Rush's phone rings both ways, just like everyone elses.