Guessing Game

Charlie Cook, a longtime friend of the Swing State Project, picks up this very interesting piece of chatter:

A fellow who oversees lobbying in all 50 states for a major corporation recently told me about a certain Republican U.S. senator up for re-election in 2010, someone generally regarded as fairly conservative who might face a serious challenge from a very conservative fellow Republican. The incumbent has not been tainted by scandal, has never embarrassed himself by making a major mistake, is highly regarded in Washington, and is considered a very effective senator.

Who might we be talking about here? The only GOP Senators discussed as potential recipients of a serious primary challenge thus far have been Arlen Specter and David Vitter, and Cook is clearly not talking about them here (indeed, he explicitly rules Specter out in the next paragraph). Any guesses?