I doubt Rush is endorsing Romney. Rush agreed with a couple of soundbites on his show of a Romney speech when he said the right things.
Well, one thing I learned listening to Rush is that he doesn’t “officially” endorse but he might as well as his daily monologues are very clear without him specifically saying “I endorse Mitt Romney”.
It would be a way for a popular host to exert his will without totally offending the sensiblites of a significant portion of his audience.
As we see here, we’re still offended and understand where a host stands but it assuages the potential uproar somewhat.