Watch this number - The match-up polls are bouncing in the margin of error, but historically a President will get a popular vote percentage that's close to his approval percentage.  Today Gallup reported that Obama is underwater with 46% approval - 49% disapproval.  Rasmussen has O's approval number at 47% and 52% disapproval; none of the recent polls show him over 49%.

Extra - And then there's likely voter party affiliation.  That's a good 'un, too.