Someone please respond: I have heard many times that if the incumbent isn’t at 50%, he’ll never win. 50% of what? Favorability? 50% job approval? 50% Obama vs. 48% Romney of whatever? What is that 50% rule of thumb?
50% voter preference. The theory being that an incumbent is already known to the voters and that the so-called "undecideds" are withholding their vote because a.) they are less than satisfied with the incumbert and are b.) actively considering the challenger.
As a consequence, the vast nmajority (2/3 or more) of "undecideds" will break toward the challenger once they are forced into making a decision.
Thus, an incumbent who cannot muster a 50% share of voter preference is unlikely to ever reach that level -- and will likely lose.