Cornyn's problem is that he offers no enthusiasm for the Republican base -- no exciting rhetoric, no red meat. Bill Clinton's TX base will turn out in mass for Kirk no matter what. Some "moderates" will support Kirk to "prove" that they are not "racist," as happened with L. Douglas Wilder in VA in 1989.
I don't recall much red meat coming out from Kay Bailey Hutchison's successful campaigns. Cornyn is going to do fine. He hasn't even started campaigning yet.
The Democrats don't really think that a black big-city mayor is going to be elected US Senator from Texas. They merely want a token black and a token hispanic on the statewide ticket so that they can increase voter turnout for their real goal, the Lt. Gov position.
I don't have any doubt that Cornyn will win.