Everyone whose inclined to be persuaded by Graham is already in the Romney camp (except for the mouthbreather anti-Mormon types, and not even Graham could change their minds.)
However, Romney's done a good job of getting the votes of a lot of moderates who are turned off by associations with the religious. So to them, this is a little like Michael Moore doing full page endorsements for Obama. Not too helpful.
Now, I love Graham and all he's done, and I'm as far to the right as anyone you'll ever meet, but this is going to be a close election, and everything is on the line.