He can beat McCaskill. McCaskill is not his problem. His problem is that he has defied the GOPe (I'm looking at you, Tokyo Rove and Haley "Buy Your Pardon Here" Barbour) and they are out to destroy him, even if it means sacrificing the Missouri Senator seat.
We really could do with different candidates and if we could get both out of there and replace them with people who don't play footsie with Democrats, I"d be fine with that.