Considering that Rush has more than 20 million loyal listeners, does Obama really want to take on Rush?
Obama needs to stir up as much hatred as he can to activate his base, who are not feeling they are getting the sort of change they hoped for.
Zero can't afford to miss an opportunity to jump on anyone disliked by the left, even Rush. Rush's listeners are probably going to vote without being stirred up. Obama's base, not so much, hence OWS & the 1%, class warfare, attacks on Rush, etc.