Oh sure -
Partisan sentiment it is, I still have to agree with the poster above...
I can't see how anyone who might have otherwise either stayed home or voted differently suddenly either deciding to caucus or support a certain candidate because of an endorsement backlash.
I'm highly suspicious of anyone voting the affirmative based on a paper's endorsement (at least for Pres... I don't rely solely on - but do read and use endorsements waaayyyy down ticket) - even more so suspicious of a negative endorsement effect.
FWIW, I do think the upside here for Obama is give a cutesy thanks-but-no-thanks to endorsement... and even as a supporter, I'd like him all the more if he did so... but I suspect that the larger thematic tenor of his campaign makes that a tightrope not worth walking.