Tom Coburn of Oklahoma might be a good choice, or Mark Sanford of South Carolina.
Romney would probably drive away more voters than he would attract. If McCain's main problem with the base is the perception that he is a RINO, it doesn't help to pick another RINO (with a worse record on important issues) as his running mate.
I would think Michael Steele would bring in some black votes but on the other hand the loonies have a way of destroying a black who does not cow tow and dilly dance to their image of the poor under class woe is my please help me liberal massa bullshit.