Charlie Cook has an interesting column about some challenges facing the Republican contenders in '08. Still, this part of his argument was entirely unconvincing:
The fundamentals are quite clear. Four out of five times in the post-World War II era, the party holding the White House for two consecutive terms failed in their attempt to win a third term. In 1960, 1968, 1976 and 2000, the party occupying the White House saw its string end with two terms.
Well, okay. Or you could say that the score is really 2-1-2 rather than 4-1. 1960 and 2000 were ties that saw the outcome go against the incumbent party. From any sort of political science perspective, it is absurd to argue that this history gives us even remotely decisive answers.