I think it's a bit of a stretch to say he's trying to harm Bush electorally. He may be trying to prod him policy-wise, but I can't imagine his problems with Bush running that deep.
Casual, non-political, I mean people who listen to be entertained, or to listen to the parodies, or want to become politcally aware but aren't the political junkies of FR.
There are still millions of people out there who when they hear Nixon, they say to themselves Watergate.
But Rush's thesis of the "big theory" has already been discredited. Political times are different and there were different issues than there are now, but Nixon didn't lower taxes etc.etc..
JMO, trying to compare Bush to Nixon, is a poison pill(the stigma of Watergate).
One could say that Reagan followed the Nixon model. He compromised on issues with democrats(such as a tax increase in 82), when the times got tough in Lebanon, he pulled out, etc.etc..