I think Rush knows that Perry is no rock-ribbed conservative. But for him it's always about winning. He believes Perry to be only one who has a chance to beat the uber-RINO Romney, and bases that the former's campaign war chest, both present and potential. I disagree with El Rushbo on that score, but I'm pretty sure that's where he's coming from.
Rush rarely pushes underdogs (like Cain or Bachmann), especially in Presidential politics. He's an establishment guy, but hopes for the most conservative establishment candidate possible. ....or more accurately, least liberal. And as far as the present establishment candidates go, he considers Perry to be that guy.
Then El Rushbo has missed the revolution train. That way of thinking is a defunct mindset that got us into the condition we're in today.
This isn't sports. It's life and death. We either get someone into that office who is driven to rein our government back into its original constitutional constraints, or it'll be up to The People to do so.
And that is going to be a nasty experience for everyone - including Rush.