What do the results so far tell us about Clinton and Obama as general election candidates?

Finally, there is one clear and consistent scenario in which primary contests provide telling clues about the fall: when an incumbent president faces a meaningful challenge for re-nomination within his own party. No president in modern times has ever survived such a challenge to win in the fall. Lyndon Johnson abdicated in 1968; Gerald Ford fended off Ronald Reagan in 1976 but lost that fall; Jimmy Carter beat Ted Kennedy but was swept away by Ronald Reagan four years later; and even George H.W. Bush, embarrassed in New Hampshire by Pat Buchanan, though never seriously threatened by him, had to invest so much time shoring up his base that the episode helped lead to his defeat in 1992.

But the main lesson is that in searching the primary terrain for general-election hints, tread very carefully. As a rule, what happens in the primaries stays in the primaries.

Correction, March 11, 2008: The original sentence misspelled Macomb. (Return to the corrected sentence.)