His statement this week saying that he would not do anything that would harm the GOP.
He's saying that even if McCain is the nominee, he will vote for the GOP standard bearer.
Can you give any more of an endorsement than that?
That is not an endorsement. The difference being that Rush will not do something to harm the GOP. It would be nice to believe that the three candidates left in the race will also not do anything to harm the GOP. One of them, McCain, has already harmed the GOP.
"He's saying that even if McCain is the nominee, he will vote for the GOP standard bearer."
That is not an endorsement. It only means that he will vote for any of the three who wins the nomination. At this point the nominee is unknown, thus the statement amounts to endorsing an "unknown."
"Can you give any more of an endorsement than that?"
Yes, but only if at this point in the process he unequivocally endorses a specific candidate. And he will not do that.