I'm thinking that if Obama wins by a plurality in which 60 percent or more of the votes cast were AGAINST him, it would embolden REAL conservatives in Congress to fight liberal nominees "tooth and nail." Right now, they're too timid because they think it's safer to be moderate; Romney's election would only confirm the advantages of being moderate. Romney's DEFEAT at the price of a really weakened Obama victory, would confirm that the safest thing is to steer HARD RIGHT.
Of course it's all speculation, it's all a gamble, but it seems clear to me at this point that the much better bet would be to gamble on conservatism by rejecting Romney.