While I don't know that much about AG Cox, I have been impressed with him during the couple of times I've heard him speak. I wish he would consider running but I've read that he isn't.
Do you know anything about Butler?
I am almost embarrassed by the lack of national level Republican talent here. Rove and company certainly aren't adverse to pushing electable talent and the silence is discouraging.
All I know about Keith Butler was from his website and his speeches. He has some recognition in the Detroit area and does write some guest columns for the Detroit News.
IMO - Candice Miller and Mike Rogers would take Stabenow out, and maybe Granholm.